ok, my friend and I did his platform last weekend, I will build mine next week. we used a 2x4 sheet of plywood, 5/8 inch thick use a high grade board, the lower grads are not as strong, more void, knots and splits in the inner layers on low rated plywood. We tested the wood at the store by supporting the edges on wood and standing in the center, the wood we choose hardly deflected, but boy did the 1/2 inch low grade stuff bend. this sheet of good stuff cost near $25 bucks, a bit pricey, but well worth it over the lower grade thin stuff.
we kept the two foot length, and four foot width, but trimmed the two corners with a rounded notch to clear the wheel well bumps.
we got some 2x2 inch angle iron, about six inches long, two each, and drilled to clear the stock seat back mount bolt, and then using carriage bolts two each side attached the angle to the wood. we removed the rear area carpet and pad to assure our new board will be dead level even with the cargo floor (we them padded and carpeted the board to give it the exact same level as the padded and carpeted cargo area. (if you want this platform to be a bed, you want it at the exact same height as the cargo floor )
at the front end of the board, we added some 2x4 and shim wood attached to the underside of the board as legs that rest on the steel cross member of the floor that the old seat bottom rested on.
it is very nice. we will next add an eyelet to the board and floor (footwell area) on both left and right side to allow the board to be latched down with a carabiner so that the board will contain our under board storage of heavy tools, as a roll over protection so the tools dont go flying about.
I might be able to supply photos later, not now, and a sketch of the angle iron hole placement for the hinge bolt. the exact hinge bolt placement on the angle iron is important as that determines the exact height position of the board relative to the cargo floor, this actually took the most time to measure and get right, and is why we had to pull the cargo floor carpet and pad to measure the cargo floor exact height relative to the hinge bolt mounting holes in the body.
On my four door xj, I obtained front seats from a two door, and these seat fold forward, thus giving me a little more length to sleep. I am 5 ft 6 in, and can sleep fine back there.
anyway I probably can get you the diminsions of the angle iron holes and photos of the board next week, as I am now in process of getting ready to leave on a jeep trip tomorrow. I dont have photo post privilege here, so you will need to PM your email to me, then I can send you photos, but again, probably not till next week.