Well cutting the hole is one thing. My question would be when you cut through that deck is it solid glass or foam cored? I think it's foam cored. If you see foam then you need to dig out the foam at least a half inch farther out than the mounting holes. Then back fill that with a blog with some mill fiber in it. Let it kick and then drill your mounting holes. This keeps the water out of the foam ensuring that you decks will remain solid.