Epoxy or polyester resin will do the trick. Gelcoat is a form of polyester resin. If you do epoxy, make sure it's quality marine-grade like West Marine or Miapoxy and not 5 minute LePage stuff.
If you know any sailors, talk to them and they can probably hook you up.
There's also a form of epoxy called marinetex. It's white, very hard, thick, comes in small quantities and is easyish to work with. If you wanna avoid having to buy like 1 L of epoxy, that could be the way to go.
Check out a hardware store, and if not a marine store should have what you're looking for.
Or you could send it into a shop and it might end up cheaper than buying the stuff yourself. This stuff tends to be extremely pricey and if you're only using a small amount, it's sorta wasteful to drop $30 when the shop could do a nice job for $10.