I created a roller on the back of my raft trailer by using wood closet rod and PVC pipe. I used the largest diameter (1 1/4", I think) closet rod I could find, figuring it would be most durable, and bought PVC pipe to match. The PVC pipe fits over the rod and the PVC pipe rolls while the closet rod does not move. Mounting brackets made from pieces of 2" x 4" attached to the back of the trailer with lag screws. I used a hole saw to make holes for the closet rod drilled a hole in the bracket and closet rod for a set screw on each end. Total cost was about $20. I can add pictures if anyone is interested.
This system has more friction than a system that uses bearings, and probably less durability than one which uses steel instead of wood and PVC. However, it has worked well for me loading and unloading a 13' Riken raft w. NRS frame, and makes a huge difference compared to loading and unloading before I built the roller. I should probably add that my trailer is 6' wide--if I used this system on an 8' wide trailer, there might be some bowing of the closet rod when a larger raft rolls over the roller.