I think I'd drill a 1/4" hole at both ends of the crack, and use an epoxy to make the repair.
"How to repair cracks or gouges in fiberglass:
-Product: Marine-Tex Epoxy Putty 'The Mighty Repair Kit' White
-Materials: Solvent for cleaning (acetone or lacquer thinner work well), 80-100 grit sandpaper, clean mixing board, two mixing sticks and a trowel
-Conditions: Minimum 60˚F temperature, 48 hours to fully cure at 60˚F, 24 hours to fully cure at 70˚F
-Read instructions on product packaging. Prepare surface for bonding by checking fibers of glass for any deterioration. Dig out any weak material until solid fibers are reached. (If dealing with a hair-line crack, use a Drimmel tool to 'V' the crack. This will increase surface contact of the epoxy to the repair, creating a more secure application.) Tape off the areas around the crack or gouge, leaving open a perimeter of at least 1/4" around the entire repair for the epoxy to overlap. Clean the entire area with a wipe of solvent and lightly sand the perimeter area, clean again. Mix Marine-Tex White in the proper proportions (5:1 ratio by volume) and apply to the surface. Be sure to overlap the epoxy on to the perimeter area so that a water tight repair is created. Water or a piece of plastic wrap can be used to smooth the epoxy. This will save time in sanding later. Allow epoxy to fully cure. Remove plastic and/or tape from the surrounding areas. Clean and sand the Marine Tex if applying a coat of paint to the surface. (Marine-Tex does not require a coating for UV or waterproofing capabilities. Any coating would be for cosmetic reasons only.) Follow specific 'gelcoat' directions if applying a polyester gelcoat to the surface of Marine Tex."