I think you should be fine mounting the amp any way up, its just solid state electronics.
If you couldn't the manual should say otherwise....
Just don't cover it up too much with the panels.
As to the actual mod, if you pry out the panel you WILL snap the tabs that are on the back of the ****-bin. The tabs are little plastic loops and the fasteners are plastic wedges. They can't be popped out by any kind of prying from the front. To remove them without breaking them you have to remove the entire inside panel so you can stick your arm back there to pop them out.
There is tons of room back there, on the driver's side where the jack is I stuffed in two ratchet straps, and spare belt (under the 4x4 module) and a first aid kit and some hand warmers near the jack.
To do this mod right, I think you need a door where the panel was (see idea below) and some type of plug up over the wheel well because if you shove stuff over it will slide down into the abyss on the other side.
Door idea: Go to the junkyard and cut a piece of matching gray plastic from a flat area of a panel out to fit where the hole is, get some small hinges and rivet them to one side, and make a latch with a knob and a piece of springy steel that can be turned to latch it shut. (this makes sense in my head, IDK if I'm explaining it well enough)
My addition to this idea is (well in a sport anyway) to cut an access door below the rear ashtray. You would have to make some kind of liner to prevent stuff from falling into the side of the body or seatbelt mechanism, but there is tons of space in there too, you could get a fire extinguisher and some ropes or whatever else in there.
I'm just scared to cut up my interior, so I haven't done it yet...
