To gain clearance to the bed rails, I made a perimeter frame with interlocking notches out of 3/4" 1x6 treated wood. Painted the top side of the wood with a can of undercoating I had laying around to provide a little protection to the metal. In the center of the frame, I put 3/4" rigid foam board. The wood prevents crushing of the foam when I load the camper, the foam board provides support and insulation under the rest of the floor. Been using it for 5 years with the camper sitting on it for 500+ nights with no problems.
