The issue was always trying to get all the "stuff" in the bed and still be able to get to it. I also needed to preserve the ability to carry 4x8 sheets of plywood, drywall, etc., without taking the shelves (and everything else) out.
You can see the strap between the upper partitions on the left. This is 1" nylon webbing and plastic buckles/keepers that can be found at most fabric stores or REI. The supports put some pressure on the floor, but they are designed to support the shelves by cantilevering off the side walls. Note that I cut the bed liner down to allow the supports to be firmly pressed against the side walls.
Everything packs in nicely, with only a few items on the floor that would have to either be removed, or taken out and placed back on top of the 4x8 sheets.