I have had rails for over ten years on my '96 2500. I have used 2x6s laid flat side to side. I put two in front of the wheel wells, one between the rails, and two behind the wheel wells. I have used other methods but always came back to the 2x6s.
I now have a '04 3500 with rails. I cut two horse stall mats in about 12" wide strips. I place them side to side like I did with the 2x6s. The mat was 3/4" thick, so I stacked two pieces. Although a bit pricy, about $90, they are the best. I leave them in withe 5er in tow. They weigh enough the wind does not blow them out. They do not slide, nor does my fiberglass bottom TC. Nothing slides including lumber. Being strips, they are easy to remove from the truck.
Wayne
