you can attach PVC trim to the exterior surface using screws and butyl rubber mounting tape. I prefer to add some blocking to the interior surface for the screws to bite into or else you can thru bolt.
Goop brand Marine adhesive will also adhere to both the smooth outside surfaces of the
fiberglass and also to the rougher inside surface as well. Be sure to remove any wax or polishing products from the exterior surface you are bonding to if you are going to use adhesive materials.
Of course there are also the Sikka Flex adhesives, those will also work. Just do the steps for installation the same way they install the port hole
windows on boats.