I did mine a couple of years ago, hard to find one that fit right. If it's too tight, you can't slide it in, if it's too loose, it doesn't fit right. Make sure it's a little longer than you think it needs to be, somehow it "shrinks"!
My rubber rail insert sometimes comes out of the metal extrusion when I "kiss" the dock a bit too hard while docking in wind. That's easy to fix, however, as I just push the flange part of the rubber part back into the metal part using a big flat-bladed screwdriver. Work altneranately on the top and bottom part of the flange parts, pushing them inward and under the metal extrusion.
Some folks have done this using a heat gun (or hair dryer), but I've not seen the necessity of using heat to soften the rubber. Maybe during a cold winter, though?
I've thought of adding a flexible sealer like a black windshield calking compound to help glue the rubber part into the metal part, but always been afraid of making a big mess that would be hard to deal with later when something required disassembly of the rail. I suppose it would be smart to do a two foot section first and see what, if any, problems develop.
For replacing the rubber insert and/or metal part, the C-Dory factory has been able to supply the parts necessary. See the following discussion: Rub Rails and End Caps.
Just some input that takes the discussion line in a direction that may be a little different - >
On a previously-owned non-C-Dory boat, I substituted a color-coordinated line for the rubber insert on the rub rail. It looked sharp and it seemed to do the job. The trick is to get line with a large enough diameter so that it is both tight and bulges out enough to act as a bumper.
It really is an old-fashioned rub rail set-up. therevdr on Driftwood Drei