I've found that good old uhu glue works well, made sure that I used some abrasive to "key" the surface beforehand.
Hope this is useful 😊

I have used liquid nails which works to glue either timber, foam or plastic (HIPS) to fibreglass. It is cheap and within our Association there are boats with fibreglass hulls over 20 years old still sailing. Their decks have been glued with liquid nails.

I normally rough up the surface to be joined and use epoxy
AustinG