On my sport trac, I have a Yakima Megawarrior (measures something like 48x50" off the top of my head). I mounted mine directly to the factory roof rack. If you have factory crossbars, it will come with the hardware to mount it. I mounted the tire with a yakima spare tire carrier.
I like it because it is the perfect width, perfect length (you can buy an extension for it too), it's solid (I can walk around on it no problem), no rust as of yet. Easy to find light mounts, hi-lift mounts, shovel/axe mounts, etc.
Ordered from here
http://www.orsracksdirect.com/
Coupons here
http://www.retailmenot.com/view/orsracksdirect.com