When I was a seviceman for a new home builder ( many years ago ), I had a similar set up in my pickup. I had 2 sheets of 1/2 inch plywood with 2x6 framework between them ( 5 1/2 inches ). In this area I made 3 4foot long drawers that slid out the back when the tailgate was down. In the front 4x4 area, I had a door on top that allowed for storage of tools that I wanted with me, but didn't use very often. Total height was only 6 and 1/2 inches off the floor of the truck bed, which still allowed me to carry full sheets of drywall, plywood ect. I could also set up a full size air matress on top of it, with plenty of head room - for weekend get aways. If I needed the truckbed empty, I would slide the drawers out and put in my garage, then, with a hand from a buddy - the complete framework would slide out - within 5 minutes everything was out of the truck. Worked awsome for me and a few of the trades on site did similar setups after seeing my setup. Never had a problem with it and worked really well. The guy that bought my truck off me asked if I would leave it in the truck - and he put it to use. Sorry - no pictures - was before digital cameras.
Tracker (Don )