This is a project in progress, and I will update this thread with pictures as I go along. The first thing to start with is to determine the maximum size of the space available to install this access hatch. The frame of the hatch will go into the area between the studs. Here is the first picture:
http://www.explorerforum.com/photopost/data/503/Inner_outer_frame.JPG
There are a few details which are important to note. The first one is that you have to miter the corners of the frame to keep everything properly aligned. I made the inner part by just connecting the 2X4 pieces together without mitering the corners, and it was a little twisted. I had to start over again, and miter the corners. They came out perfectly even after this. This is a close up view of the mitered corners:
http://www.explorerforum.com/photopost/data/503/Mitered_corners_.JPG
The next thing to note is that the lower frame has to counter sink into the area between the beams. The upper 2X3 frame has to be slightly higher to allow a space to install weatherstripping. Here is a picture of this:
http://www.explorerforum.com/photopost/data/503/The_outer_frame_is_higher_for_weatherstripping_.JPG
The top, and bottom sections have to be connected on one side with a piano hinge. A regular hinge is too bulky. This pictue shows how the upper half will look when it's in the open position:
http://www.explorerforum.com/photopost/data/503/Piano_hinge_top_view_.JPG
This is how it looks on the bottom when it's in the closed position:
http://www.explorerforum.com/photopost/data/503/Piano_hinge_bottom_view_.JPG
This is it for now. The next part will explain how to make the top part.

I didn't think that this project would be on hold for such a long time when I first started it. Sorry for the delay. This part will show you how to construct the top part, and apply metallic foil to protect the wood from the elements. Here is the top piece of wood getting screwed down to the frame:
http://www.explorerforum.com/photopost/data/503/The_wood_top_is_getting_connected_.JPG
The screws are galvanized deck screws. Here is the top piece of wood fully connected:
http://www.explorerforum.com/photopost/data/503/The_wood_top_is_connected_.JPG
One corner was sticking out just a drop, and needed to be planed down:
http://www.explorerforum.com/photopost/data/503/One_edge_needs_to_be_planed_down_.JPG
Here is what the back of it looks like:
http://www.explorerforum.com/photopost/data/503/The_back_of_the_skylight_with_the_wood_top_.JPG
There is a plastic hemisphere going on the top to enable light to go into it. I'm using it as a template to mark where the cut has to be:
http://www.explorerforum.com/photopost/data/503/The_hemisphere_is_used_as_a_template_.JPG
The Rotozip is going to make this cut:
http://www.explorerforum.com/photopost/data/503/A_Rotozip_is_cutting_the_top_out_.JPG
It was hard to cut this without breaking about a half a dozen bits. The left side appears to be cut wrong in the picture, but it's only a scrape mark in the wood. You could see the part that was cut in the background. It has a straight cut, but is a little wobbly. The cut on the right came out a little better since it was done after the first one:
http://www.explorerforum.com/photopost/data/503/The_top_is_cut_off_.JPG
This is the aluminum foil tape that is going to be used to weather seal the wood:
http://www.explorerforum.com/photopost/data/503/Aluminum_foil_tape_and_a_box_cutter_.JPG
Here is what it looks like as it's being applied:
http://www.explorerforum.com/photopost/data/503/The_tape_is_being_applied_.JPG
Another piece was applied, and smoothed out:
http://www.explorerforum.com/photopost/data/503/The_tape_is_smoothed_out_.JPG
Here is the top with the tape on it:
http://www.explorerforum.com/photopost/data/503/The_top_is_covered_with_aluminum_foil_tape_.JPG
The next part will show the hemisphere being connected, and being sealed with adhesive. There is a gasket which has to go around the inside to seal it completely. There are two sides which lock in place to keep the lid open, and two pins to lock it. Those will be the next installments for this series of instructions.

It's time for the hemisphere to get mounted to the top lid. The base has to be marked before it could get connected. The hemisphere will be the template for the line:
http://www.explorerforum.com/photopost/data/503/Mark_the_place_where_the_hemisphere_will_go_.JPG
This is a good grade of silicone which isn't cheap:
http://www.explorerforum.com/photopost/data/503/Marine_grade_silicone_sealant_.JPG
A bead of silicone will get applied to the base before the hemisphere is mounted:
http://www.explorerforum.com/photopost/data/503/Apply_a_bead_of_silicone_to_the_base_.JPG
The base has a bead of silicone all around the opening which will come into contact with the rim of the hemisphere:
http://www.explorerforum.com/photopost/data/503/A_bead_of_silicone_is_all_around_the_opening_.JPG
These are the screws for mounting the hemisphere to the base:
http://www.explorerforum.com/photopost/data/503/Truss_head_self_drilling_screws_.JPG
It's a good idea to let the hemisphere sit for about an hour before screwing it in since the silicone could compress itself out from under the rim. This grade of silicone is very thin. After an hour, the screws are ready to go in. They have to have close spacing:
http://www.explorerforum.com/photopost/data/503/The_screws_are_getting_mounted_to_the_rim_.JPG
The screws are completely around the hemisphere:
http://www.explorerforum.com/photopost/data/503/The_screws_are_all_around_the_rim_.JPG
The screws will get covered with aluminum foil tape:
http://www.explorerforum.com/photopost/data/503/The_aluminum_tape_is_covering_the_rim_.JPG
The top of the hemisphere is sealed:
http://www.explorerforum.com/photopost/data/503/The_aluminum_tape_is_around_the_hemisphere_.JPG
This completes the top part of the hatch. The next part is about sealing the under part with a gasket, and connecting the lid support & locking mechanism.

Lol. At first, I didn't realize what forum this thread was in. The whole time I was reading, I kept thinking "This guy is freakingnuts! Is he really going to cut a hole to mount that in his Explorer"?

It's for a house, garage or a storage shed. I didn't even think of using it on a roof of an Explorer. Maybe it could work on the top of a motorhome? I'm making this for a shed in my backyard.

I'm a bit confused about what exactly you're going to do with this. Is it just a "skylight" for the shed? What's the hinged piece for?

It's a combo skylight/roof access hatch to enable access to the roof. It has a clear dome to enable light to enter. The hinge is connected to the top half to enable the top part to swing open.

Hmm, interesting. Now I am interested to see it finished and sealed up!

I still have to install the rubber gasket, hatch supports, and locking pins.

Part IV completes all of the assembly instructions. These are the basic things which are needed to finish this project:
http://www.explorerforum.com/photopost/data/503/Skylight_weatherstrip_mounting_hardware_and_handles_.JPG
The mounting hardware has installation instructions on the back of the packages:
http://www.explorerforum.com/photopost/data/503/Skylight_hardware_instructions_.jpg
The paper on the far left is for the locking pins. The center paper is for the lid supports (left & right), and the one on the far right is for the grab handles. The roll of weatherstripping was just long enough to complete this project. It's 17 feet long. The first strip is getting nailed down:
http://www.explorerforum.com/photopost/data/503/Weatherstrip_.JPG
The corners were cut with an ordinary scissor, and mitered:
http://www.explorerforum.com/photopost/data/503/Weatherstrip_single_miter_.JPG
The corners are joined together:
http://www.explorerforum.com/photopost/data/503/Two_mitered_corners_of_weatherstrip_.JPG
Here is the first outer row of weatherstrip:
http://www.explorerforum.com/photopost/data/503/A_single_frame_of_weatherstrip_.JPG
The second row is mitered, and now being connected:
http://www.explorerforum.com/photopost/data/503/Weatherstrip_double_mitered_corner_.JPG
Here is the whole frame with the double weatherstrip:
http://www.explorerforum.com/photopost/data/503/A_double_frame_of_weatherstrip_.JPG
Now it's time to connect the lid supports. Here are the left & right parts:
http://www.explorerforum.com/photopost/data/503/Two_lid_supports_holding_up_the_lid_.JPG
Here is a single side:
http://www.explorerforum.com/photopost/data/503/One_lid_support_arm_.JPG
The two sides require locks on the outer corners. A hole has to be drilled through both pieces for the pin to get inserted:
http://www.explorerforum.com/photopost/data/503/A_hole_is_made_for_the_locking_pin_.JPG
This is what it looks like after all of the parts are installed:
http://www.explorerforum.com/photopost/data/503/The_locking_pin_in_its_holder_.JPG
Here are the two corners with the locks installed:
http://www.explorerforum.com/photopost/data/503/Two_lid_locks_.JPG
The two grab handles are the last parts to get installed. Here is the inner one:
http://www.explorerforum.com/photopost/data/503/The_inner_grab_handle_.JPG
Here is the outer one:
http://www.explorerforum.com/photopost/data/503/The_outer_grab_handle_.JPG
They are connected with two bolts going through each other. The hatch cover is sandwiched between them. Here is the completed bottom view:
http://www.explorerforum.com/photopost/data/503/The_completed_bottom_view_.JPG
Here is the completed top view:
http://www.explorerforum.com/photopost/data/503/The_completed_top_view_.JPG

Here are some pictures of the hatch installed on the roof:
http://www.explorerforum.com/photopost/data/503/The_hatch_is_open_.JPG
http://www.explorerforum.com/photopost/data/503/The_hatch_is_closed_.JPG