Foam Board or Rubber Mat For Under Camper?

I have not had the plywood slide backward, but it you look closely at the picture, my plywood is cut around the tailgate opening such that it is locked in by the tailgate frame. Actually it's pretty hard to see in the picture but it is there. It can't slide backwards. I think I would worry about this otherwise, particularly since it is on a poly liner which is a bit slippery. Still, the camper is held forward by the tie downs so I don't think the plywood would try to crawl out from under.

I think the Bigfoot decal is due to all the screws underneath. These will for sure tear up the paint on the bed. If you use plywood and no rubber, you will get a nice imprint of the screw heads in the plywood in very short order.