Ahh... If it's fiberglass then I'd use Marglass. It's basically Bondo with fiberglass reinforcement. It comes in different length strands, the longer the strand the stronger it is, but harder to work with.

If Bondo takes more than 5 min to dry get it off there and do it over because it will never dry, and if it does it's not going to be very strong. You want a 10-1 ratio of filler to hardener with any of these fillers, including Marglass.