well, I'm headed to Texas at the end of the week from B.C Canada, and I needed a place to sleep, the back of the truck sounded great!
I didn't want a super enclosed frame that was hard to see into, because I have to cross the border, and that may have been a hassle.
here is the frame, I used a thin piece of plywood on the bottom for rigidity.
I used 1- 2x12x12 piece of wood for the side rails, cut in half to fit the 6 foot bed.
I had some cedar 2x4 lying around, so I used that to make it even tougher,
I used 3 pieces laying horizontally across the top, to maximize storage space underneath, and at the front near the cab, I put 2 pieces vertically, again for rigidity.
I also wanted to be able to take the top sheet off easily for when I get to the border. I used a 4x8 foot piece of 5/8 plywood for the top, I wasn't sure it would be wide enough, but as you can see there is only about 4 inches on either side of free space, which leaves room to stuff things down the sides for storage.
To secure the top sheet from sliding around, I drilled 2 holes at the back end, through the top sheet and into the 2x12, then I cut some big stakes in half and cleaned up the ends. I can easily slide them in an out to remove the top sheet.
by using the 2x12, I was able to maximize storage underneath, giving me a total of 10 inches for my suitcase.
more headroom would have been nice, but I needed room to store my stuff while sleeping (gf will be with me, so the bags have to go underneath).
well there you have it, I may cover it with some outdoor carpet before I go, we'll see.
hopefully this helps the next guy in building a custom sleeping platform.