The rigid pink insulating sheets sold at Home Depot work very well and come in 1/2", 1", and 2" thicknesses. With a 1" sheet and my 3400 lb. camper there was no compression of the sheet. It cuts with a box cutter and weighs next to nothing and it will not absorb water or warp like a plywood sheet will do.
I have the pink sheet on the bottom and then a rubber mat and then the camper. No sliding at all. When the camper is off the truck I leave the pink sheet and the truck mat in place. The rubber bed mat weighs a lot more than the pink insulation.
