My latest installation was on my penthouse fiberglass top. It also is ribbed. It had the additional problem in that there are 5 strips of plywood sandwiched in the fiberglass that run fore and aft. Putting the vent hole in the roof cut one of these "rafters" in two. So I had to make a frame to sister the cut rafter to the ones on either side of it to assure I got the roof strength back. I also used rope caulk to seal vent to frame joint and the frame to roof joint. I have had no leak issues with the installations.
You will note that with this design I added fillers for the roof rib valleys. It was easy to do since I was all ready making a frame to strengthen the cut roof.
You can see some of the rope caulk squeeze out at the joints. I have also painted the wood frame white to protect it and match the penthouse color since installing it.