Closed cell Foam on Bed

Thanks for all the replies so far , would still appreciate any more . I actually need about 3 inches to have clearance of the camper over hang to top of cab . I have bought 2 sheets of 2" , 4x8 foam insulation . I too have fiver hitch rails to deal with . I was thinking about setting the foam sheets in the floor of the bed box ( tail gate down, 6.5' box) and then letting the camper down on top of the sheets . On the bottom of the camper there are 4 rails which run across the outside floor that may groove into the sheets. I also feel that the fiver rails may groove into the bottom sheet . Do you think this will prevent the sliding or not?I am not off roading at all. Open to all suggestions and previous experiences. Thanks