Well technically just as the raw 4x8 i think the plywood comes it wont fit, but once cut you can get the
front piece which is the entire 48" to fit diagonally. That's if you notice i made it 2 pieces held together by some hinges so i could still access the spare tire compartment.
The front fattest part is 48"
The smaller back part is 31"
And the whole thing is 48 1/2"
But to get a easier general idea of how i did it, when you're at the photo bucket image try to view the album itself and you will see the "frame" of it i guess you could say. Their is one 2x4 width wise on the very front so it has something sit on and support, and two 2x4s length wise so that the back doesn't just cave in. The front is very sturdy if you still have the middle support for the rear seats also, nothing is caving the front in believe me.
They rest on the back plastic piece for now since i ran out of 2x4 but i will be reinforcing it asap, but I will say for only resting on the plastic it supports my 145lbs very easily so not really to much of a worry.
So all in all you need: 1 sheet of plywood, 3 48" pieces of 2x4, 2 door hinges(if you do what i did), some upholstery glue, and in my case some cheap carpet from Walmart(it looks fine i promise)