Welcome to iboats!:welcome:
I doubt they are made that narrow. The center section of a 5" roller isn't much wider than the center section of a 12" self-centering roller anyway. Get polyurethane rollers. They are durable, won't mark the hull, and are slipperier than rubber so you can make minor adjustments easier. Alternately, you could remove your current brackets and replace them with wider ones. Can you post a picture of the rollers on your trailer?
