I use toggle anchors behind an aluminum plate that is riveted to the aluminum skin. A variation of what is mentioned above would be to weld nuts onto the back side of the plate. If you use 3M 5200, you can glue the plate to the fiberglass wall. This will disperse the load over the whole plate area, and should make for a secure mounting point. IF you can't find stainless sheeting cheap, rummage through someones junk pile and see if you can find an old stainless steel sink. You can cut the plates from the bottom of the sink. Polish them with a wire wheel and it should look good. Plan ahead as 3M 5200 takes over a week to fully cure.
