i have a fiberglass camper shell for my truck and my truck has a plastic bed liner.
I bought some cheap carpet at Lowe's, the kind that guys often use to carpet bass boat. The carpet is a close match for the carpet that came with the camper shell. I cut a piece to fit in the bed between the hubs and bought a cheap walmart rug to place underneath for extra cushion.
I sleep on a self inflating roll up pad, it's low profile and self inflates to 3 inches and is long enough for me (i am 6'2).
Here is a link one similar:
http://www.rei.com/outlet/product/776609
I have toyed with the idea of building a bed platform just above the wheel wells and using my futon mattress, but I would loose head room preventing me from sitting up when I woke. That might not be a concern for some, but I have asthma and being able to immediatly sit up during an asthma attack is my concern.