You also need a backing plate (If the assembly does not come with one) so the
fiberglass will be bolted rigid, (sandwiched) between the outside and inside plates. This will cut down on
fiberglass flexing and will seal the outside plate tight against the butyl tape and
fiberglass.
I was going to put a door on our
Scamp 5r but could not find a suitable FLAT spot. The only places where it should go had a curvature to it and I didn't think I could get the door to seal tight and/or it would mess up the fiberglass by putting undue stress (Distorting) by screwing it down tight. Undue stress could lead to fiberglass cracking.