The main reason given for using pop rivets with
fiberglass is they will break away when they need to. If there is undue strain on a particular fastner, it is better to have a rivet pop than to tear a hole in the
fiberglass.
If using screws, be sure to use washers or some other method to avoid high stress in a small area and spread the strain to more than one fastener.
An air operated rivet gun makes the application a lot easier.
For vent covers, etc. that need to be removed occasionally, install the little inserts that look like a cocklebur and have internal threads. Look up "Nut inserts for wood". Put some epoxy on them before inserting. Use stainless steel screws.