Interesting problem.
Over a decade ago, I was whining to a friend (with a lot of diy experience on boats) about one place where the frp part of our cabin sole was flexing a bit. Just one... darned... place...
There was about an inch or a bit space more under it, above the inside of the hull bottom.
What was needed was a little pillar support, angled for the hull and perfectly glassed to the underside of the frp molding.
He listened and asked for a detail or two... and then suggested that perhaps -- with a little block of foam rubber and some 2" glass tape, and some epoxy -- I could insert the boating equivalent of a "bacon wrapped tenderloin" !
So I reached under with sandpaper, and roughed up both surfaces, cleaned it with some acetone, and inserted the foam with the cloth wrapped around it and tied with some string, after soaking it in some epoxy.
It cured and the flexing was totally gone.
Odd idea, but maybe worth a try.
Or, I have totally misunderstood the problem! In which case I will go back to lurking
Best of luck,
Loren