I have a rubber bed mat that's about 1/2 inch thick. In addition, I added a 1x4 around the perimeter of the bottom where it touched the bed. That raises it enough to clear the bed rails by about 1/4 inch.
On the front, I took the top bolt out of each of the bumpers and let them swing down giving me about 5 inches of clearance below the front rail when loading (as long as you have it an inch or so off the bed when loading). When the bumpers touch the front of the bed there's about 3/4 inch clearance between the front of the camper and the top inside edge of the bed.
I tried the stall mat that many say they use. I didn't think I was a wimp until I tried to put it in the truck by myself. They are heavy.
I also tried the builders foam some use. It holds the camper up, but the weight of the camper will crush the edges of the foam when you travel a bit putting the camper weight on the aluminum cross supports for the tanks. On the '99 model yours are wood, but they aren't designed to support the camper weight. The weight is meant to be supported by the outside vertical tub walls.
The 1x or 2x fastened to the bottom work much better.
Good luck with your new toy.