I have not had to solve that particular problem, but it sounds like a rivnut might be an answer instead of a molly-bolt
Another creative solution, might be to get a sheet of 3/8" thick G10. Epoxy it down on top of the existing fiberglass base. If you go slow and are very careful with a sharp new tap you can drill and tap it. It is strong enough to hold threads, and is non-corroding. I have used it a lot as backing plates. It usually comes in a light green color that is not horrible to the eye.