Well, after all the excellent suggestions I got in this thread, I set about building my platform concept on Friday night. Actually I mostly just pulled the other seat out (WOW that thing is a bear to get out), then took a bunch of measurements. Went inside with my sketch pad, had a cold beverage, and started working out how I was gonna do this. Started working on Saturday afternoon, my goal was to make a big flat platform with lots of storage underneath, but I wanted to use up a bunch of scrap lumber and hardware I have laying around. I ended up only having to buy a package of screws and a piece of poplar for the front trim, so I have less than $10 into this whole project. Here's what I came up with:
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The little vertical trim at the front is a lip to hopefully catch stuff from sliding off the front edge and falling to the floor. I probably will need to add traction to the deck, maybe I'll get some cheap carpet runner material and staple it down.
Hinged portion open, there is a LOT of room in there. I found a scrap of commercial carpet at the office that was heading to the dumpster, so I cut a hunk to protect the truck carpeting.
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This is how I attached to the ARB drawers, I used some Simpson angle brackets, drilled out to for the existing bolts.
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I'm bolted to the floor in 2 places, using the existing seat bolts. You can see the wires from the seat (heaters and airbags) tucked under the carpeting.
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I'm not that tall (5'-8"), but I wanted 6' of platform to fully support a good mattress. Even with the front seats all the way back, there's still plenty of room.
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This is all made out of scrap 1X pine and a chunk of 3/4" plywood that I recycled from another project, and a piece of scrap 2x4. Some screws, a few brackets, some spare hinges, and it's all done. This is actually a prototype, I really want to make a more functional unit that will look and work better, but I have a road trip planned for January, so this will let me work out ideas. My next one will be a welded steel frame, with 1/2" birch ply panels, covered in carpeting to match the ARB drawers. I also want to try to make it modular, so I can pull out or install halves that match the seats. It's back out of the truck now, and it is a challenge for one person to move.
So far I've decided the hinged lids are too big and awkward.