"Training" Fiberglass

Fiberglass will move with heat in many cases, but not far and reheating it will often send it back where it started.

Best bet is more glass holding it to the proper shape. Secure it close to your final shape and and add more glass to the backside.
Other choice is to add material and then sand/sculpt it to final shape, then refinish the surface. It's not impossible to get the materials to even patch over a gel coat layer if there even is one.

Wear a mask and treat it like wood working with extra steps ;)