The rub rail is damaged on the port aft corner, so I decided to replace that part of the stiffener as well as the entire insert.
Damaged rub rail stiffener.

The stiffener and flexible insert are listed on the Taco Web site (
https://tacomarine.com/rub-rail-store-vinyl-inserts-v12-0303), but apparently they no longer stock the stiffener. I found three online vendors that have some, but only as a single 20-foot piece, which requires an expensive shipping charge. One of the vendors (Blackburn Marine,
https://www.blackburnmarine.com/) agreed to cut up the stiffener into three pieces before shipping it. If you can’t find a piece of stiffener in a salvage yard, I recommend you consider this approach.
M