I went over the falls on one of my boards the other day and landed on one of the side fins, result - bottom of fin plug broke and damaged high density insert.
I routed out the plug and considered my options. The insert was cracked at the bottom, it just needed reinforcement rather than replacement. If I could glass the inside of the hole I figured I be good. But getting a tight lamination would be a problem.
Then I had an idea partly inspired by something Benny posted about making fin tabs. What I did was make a plug out of low density EPS that was slightly bigger than the hole. Then I wrapped a couple of strips of wet glass around the EPS plug, dropped a couple of round pieces of glass in the bottom, and carefully squeezed the EPS plug into the hole. The EPS pushed out against the walls of the hole, creating a tight bond between glass and insert material. When the glass was hard I scrapped out the EPS. Plug hole is rock solid and sealed.
My idea worked beautifully so I rewarded myself with an extra beer after dinner.