There must be a user group on the web for Sea Snarks. They have been around for a long time.
Anyway, I would think epoxy resin, not polyester resin, should be used. And don't use wood filler. Wood filler is made for, well, wood. Why not use the same epoxy resin you will use with the glass, but mix in a bunch of low density filler like micro balloons? Big name resins, like West System, work very well but are pricey. Check at your local marine chandlery like West Marine, or online. I have had good luck with the epoxy resin from US Composites in Florida.