[Ronbo]

Best practice is to not rely on the fibergass to hold the screw. Use a backing plate of sheet metal and thread the screws into this, not the fiberglass itself.

I've yet to see fiberglass not form stress cracks over time from the weight of items screwed to it, especially on boats that take a lot of pounding.

I'd also recommend fabricating a rubber gasket under the base of the mirror.