While changing pickups to a 2004. model 3500, I found my 2003 Bigfoot camper had problems with the newer cab style however ends up with the top of the door when being opened, hitting the camper cab over. I have some post where other brands of campers also need a spacer for bed rail clearance.

I remedied this by not using plywood spacers or wood frames with addtitional weight in the bed but rather went to Home Depot and got two sheets of the inexpensive blue high density 2" foam. On top of that foam I put my bed mat.

I have returned from a 14000 mile trip and they worked perfectly, no shifting, no scratching and cab door clearance is about 1/2". The 2" insulation value to the floor was a side benefit.

Hope this is of value to people needing to use a spacer for slide in campers. It weighs just a few pounds and is easy to remove and store when unloaded.