Possibilities,
Attach an aluminum plate like a piece of flat bar prior to attaching the hinge flange to the plate.
One could use a piece of teak and screw the teak to the boat. Use sheet metal screws to attach the teak. Quite a few if need be. One could attach the hinge to the backside of the wood w small bolts after routing out a slot.
Theses are excellent, we drilled holes in truck doors and used them to mount brackets for west coast style mirrors.Lots of good suggestions here. A Wellnut is my go to fastener in a situation like this. It works very well on thin fiberglass as it expands on the other side. WellNut - STANLEY® Engineered Fastening