If you have the cab clearance, a 3/8 inch thick, sticky, fluted rubber bed mat is enough. The thick ones above will allow compression and expansion, rocking of the TC body, side to side on curves or undulating road surfaces and possibly in the arc of hitting your cab. Also, to contain the beast from sliding around, I use Lance camper Guides, essentially 4 metal triangles bolted to the bed, just wider than the base of the TC to keep the box from slipping from side to side. What's wrong with that, you ask? If the tie downs are tight it puts an extraordinary load on the stretched side.
Jasan, love the power wagon, with a winch no less.
jefe