This roller is perfect. Easy to install, reasonalbly priced and does the job.
Northern Industrial Roller and Bracket Set | Roller Supports | Northern Tool + Equipment
I have a Little Tex utility trailer with 10" side rails. I had to install cross pieces (2 x 8" wood covered with carpet) in order to use it with my raft. For a number of years I just lifted and grunted my boat into the water. I installed these rollers on the rear cross piece and it slides right in. I had a plan to build a home made roller, but after I priced out the materials I found it was not worth it. Four pre-built rollers cost me $118, including shipping.