I was just looking at the Hamilton Marine catalog today and saw that they had a lot of relevant products including rubrail materials, but I don't have specific experience with any of them to make recommendations beyond potentially taking a look there.

For example: https://shop.hamiltonmarine.com/products/gunwale-guard-44358.html but they have nearly anything you can imagine.   

   You could try a couple of twin eye round fenders slung horizontally just below the rails. Just get a couple in diameters big enough to stand proud of the rails. Mine are oval at about 3” in the flatter dimension and provide barely enough padding. Done again I’d look for round in 4” - 5” diameter.