Or...patch it in smaller sections. With fiberglass you don't have to fill the whole hole at one go. Most rounded areas can be divided up and the patches easily joined one to another with mat and resin as one dries and you move to the next.
Vinyl flooring covered with waxed paper might work very well.
Paul filed a 24 x 36" hole in the side of Peanut--we had some extra panels of fiberglass so he cut one as close as possible and relied on mat, resin, sanding, and bondo-hair plus days of work to get it to fit right--and it did.
vent in port side
Inside area for repair
Outside primed after repeated sanding and fairing with bondo-hair
Final appearance of filled vent hole
BEST luck to you, it's a big job, and then suddenly it's done. Kai