I don't use fiberglass on the panels themselves.I had a wood base secured to the panel and the material stapled to that.
In places that I had to secure the material to the door and the staples wouldn't work, I use CA glue. It will not release when the fiberglass resin starts to cure like other glues will.
Now to directly answer your question, if you have a ring and you want to mount it to the door and mold that in, I would use a liquid plastic/urethane.
I use SEM 40507 1 minute set liquid plastic. It requires a specail applicator gun though.
Monday I should be using this to do two 7" drivers in a door next week. I'll take pics and make a thread.