i have an '07 ford and found that 3" foam is best to keep the cabover from impacting the truck when offroading.
The way i did it:
i used monster 2" velcro to attach 3" extruded polystryene (XPS) foam to the underside of the camper (1 less thing to deal w/ as its attached to the camper). Velcro makes removal easy (if i ever need to do it). I water-proofed using eternabond double sided tape (between foam and camper) and finished edges with 3" lexan corner guard (to protect the edges). An aluminum sheet protects the bottom side of the foam.
I plan on custom fabricating some low profile toolboxes that will act as centering guides (wheel-well and forward) and provide useful storage.