Another idea for you to be able to use NMO mounts is to make a metal channel that you can mount to the roof. We use one in our ESDA's Incident Command Center. We have 2 channels one on each side of the vehicle, and have 14 NMO mounts. I am not sure how the channels attach to the fiberglass, but I can check, and give you more details if interested.
Jonathan (N9TJX) firstname.lastname@example.org