Will the GRP take the load anyway, assuming you could fix to it, or will you have to get a fixing to the underlying structure?
Maybe piece of angle screwed to side of joist and against bottom of GRP panel, hole drilled through both and then bolted together with external penny washer and appropriate sealant. Leaves protruding stud to take panel fixing bracket? My boat has plenty of examples of this kind of seal, underwater, and good for life of boat. I hope...