I had a similar issue on our boat (1986 First 375). Our strut penetrates through the hull and up into a heavy fiberglass sleeve with gussets supporting it. It's glassed in place, with no bolts. Per repair instructions we got from Beneteau, when the boat is hauled out drill a dozen or so 1/4" to 3/8" holes into the fiberglass mount from all angles, deep enough to hit the strut itself. Let everything dry out, and blow compressed air into the holes to clear out the fiberglass chips and the last of the moisture. Now temporarily seal the hull penetration around the strut with modelling clay or whatever, and liberally refill the holes you drilled with epoxy. One of those West System syringes is good for this. The epoxy should fill the slight gap that has developed between the strut and mount. When the epoxy sets, all done.