Are you going to use that seat as a mold, or the actual seat? Thats gotta weigh a ton.
External molds are a pain. The finish comes out pretty gnarly. It adds a lot of sanding and finishing.
From my experience, it would be worth it to use your seat and create a mold from it, then fiberglass that. Then the finish work is much easier.
Actually after doing it a few times, its plenty worth it to pay 50-120 bucks for one already built.