A couple questions...
What grit sand paper do you recommend to sand the fiberglass?
Would "3M/1 gal. Bondo-Hair long strand fiberglass reinforced filler" from Autozone be a sufficient product to use to attach the board to the fiberglass?
Try 80 grit or courser. Just enough to scar an otherwise smooth surface to get a good bond.
Anything with fiberglass strand has some structural integrity and good adherence unlike a plain body filler. And, mix in a polyester resin 50/50. Don't forget the catalyst.
If its the hull surface use intermediate blocks and not the board/panel direct.