Fibreglass is the mat used in extensive jobs for reinforcement.

the "BOG" (filler) used by panel beaters is just an epoxy resin.
the auto bog is quite stiff and easy to work with and sets like a rock; can be sanded and you'll never know the repair was there if you take care.

I'd try bogging it before buying a new one - you'll be surprised how well it works if you prepare it properly.