Having a camper that isn't flat on the bottom only compounds the problem. If you have two wood spacers 8' long, filling the two valleys that are under the edge of the camper tub then you will be transferring the load to the bottom of the bed where the bed's cross members can carry the load instead of the thin sheet metal peaks of the bed. As far as type of material that will "give", I'm sure there are plenty of good choices. The one above looks like it is real good quality. The pink Styrofoam was suggested only because it is relatively tough and cheap.