Sorry I didn't make any plans for the platform build. I spent an hour just measuring and re-measuring. I basically figured out how high things were that needed to go under the bed and that dictated how high to make the platform boards. We also didn't want to be to high as to lose the space between the bed and the ceiling. Kind of a balance. We wanted to keep the rear driver side seat in place so the bed takes up as much room (width) as possible with the driver side rear seat up. We then removed the passenger seat back. (very easy) A piano style hinge was added (lowes or home depot) to allow the bed platform to increase in length once the passenger front seat is moved all the way forward and tilted up. When someone is riding in the passenger seat, the piano hinge just flips the board back over the bed and out of the way. Not shown is a PVC 1" pipe that works as a leg under the piano hinge to make the bed level. This then gives about 72+ inches of length. This pic attached shows best what the the lumber used was. I believe the platform is 3/4" sanded plywood and the extra bracing shown is 2 x 3 studs. I am not much of a woodworker and I would say most anyone with some experience using a circular saw, jig saw and a level can build it. The carpet gluing is easier then I expected. I just bought the right glue for laying down indoor/outdoor carpeting and used a medium sized putty knife to lay down the glue. Placed the carpeting on the wood and smoothed it out. Needed to dry at least in an area of 65F degree's or more. Once dry (24hours later), went back with a new razor bladed and trimmed it up. Its simple but works well. Been camping with the FJ and our van quite a few times.
On a side note, in an FJ, you are basically somewhat trapped while sleeping as the side doors cannot open (as you know) without the front doors being open first. Unless the rear window is open, it makes it hard to exit. There is a fantastic youtube video of how someone solved it. It requires drilling a hole into the rear door on the inside, removing the interior trim and adding a steel rod. We did this and this was awesome for being able to exit out the back of the FJ by just pulling up on the steel rod to unlock the rear door. I will try and get a pic of this and will try and find the youtube video of it. This was the hardest part of the build. LOL Taking my time was key and it turned out good.