This is a common problem with many production boats (you know which ones I''m referring to) that is to say, using self tapping screws rather than machine screws and nuts. If access is available on the opposite side of the fiberglass, I would substitute the existing fasteners for a screw, nylock nut and washer. Filler in the holes will not produce favorable results as the same thing will happen repeatedly. Self-tapping fasteners are a cheap and dirty way to put things together. This is the essence of "Value Pricing" everyone likes to talk about.