Sometimes the most obvious and simple things are the best option.

Poking around in the far back of the truck, where the optional 3rd row seat folds down, I discovered I already had a heavy-duty securing system with anchors into the frame, and web straps designed to secure hundreds of pounds of delicate, odd-shaped cargo: the seat belts!

I moved my big Stanley plastic job box back there pulled the seat belts across it, and secured them to each other with a heavy carabiner. Don't see how I can improve on that much.

Dave