Mine isn't nearly as well crafted as many of the setups here, but I thought I would share my setup in my Land Cruiser (93 FZJ80)
The top is 3/4" ACX plywood, sanded and primed w/ Kilz then carpeted with cheap Home Depot indoor/outdoor carpet that is a remnant from what we used for her play house. Not shown are 4 recessed tie down rings in the main section of the top (added after these pictures). The top has three hinged sections - a long wide one at the front that will be a separate storage area eventually (divider board not in place yet) and two sides at the wheel wells that fold up to allow removal.
The frame is a little unusual for a couple reasons. First that I wanted to get the height as close to 11.5" as possible so that the overall height of the platform matched the second row seats when they are folded down and
raised to flat. The second is that I wanted to have a built-in rail for a future drawer. The frame is 2x10's on edge notched for three 2x4s width wise that support the top and hinged sections. The 2x4s are about 3/4" narrower than the tailgate so that I can slide the whole platform in and out. The 2x10s sit on 2x6s which will be used as slide rails for a future drawer (using UHMW polyethylene slides) and there will be about 1.5" below the drawer to store a thin table between the rails. It is secured in the front using turnbuckles to connect the floor tie-downs behind the second row seats to lag hooks into the 2x10s. The rear is secured by removing the rear tie-downs and bolting the 2x6 through the floor to the four existing M6x1.00 thread holes. The holes are lined up with the 2x10, so I had to notch the 2x10 to get access. that also allows for the future slides to not interfere with the bolts.
I plan to add a center divider below the hinge for the front section to separate the front storage area and include a locking mechanism (probably a simple cane bolt and eye) below the main area to lock the front storage area. I normally have two car seats in the second row which will prevent the seats from being folded, so if the top can't be lifted it will be fairly secure and hidden.
Finished product works as a double wide bed, uses the second row seats folded and raised with a block of 2x4 to be flat. My daughter loves it!
Carpeted on top to reduce noise and glare in the back window. Not shown are 4 recessed tie down rings on the top added after this picture was taken.
With tailgate closed it is about 2" below the top of the tailgate, enough to keep things from rolling out.
Top of platform before side hinges were installed. The recessed tie downs are in the main/center section of plywood.
Frame as I was test-fitting, does not include the middle cross piece yet which was added to have a support at the front hinge and for the side wings to sit on.
