404 is good filler to use w/ what you're doing, & what SC84 describes is on the money.
Here is a your almost exact scenario explained on West's how-to site:
http://www.westsystem.com/ss/repairi...in-fiberglass/
Scroll down to the "Zone 3 - high risk" section. Instead of making a puck in a paper cup, you kinda just made it in place, which will work fine. Just need to grind it back some now to give yourself a clean surface for a few layers of glass.
Have you checked out how Strick (& others) mounted his seacock to a glassed over plywoood plate?