trim the hole neat so you have good clean edges. then sand the inside so it tapers to nothing (or nearly) from about 10mm. use the ally (or melamine) but give a coat wax first. then screw it on from the outside with screws that are just long enough (or grind off excess). gelcoat the area from the inside on the temp mould. when gel has gone off, apply csm and resin using a larger piece for each layer. following day, unscrew temp mould, countersink screw holes and fill with gelcoat.
jobs a goodun