I'm guessing you mean snaps like for a tonneau cover?
For spoilers (and I assume the roof racks) GM used "well nuts". Threads that have a rubber plug around them. Push it into the hole, tighten the screw into it, and the plug seats against the fiberglass, holding it in place, while sealing it from water.
You probably wouldn't want to use them on the thin parts of the shell, but where it is reinforced they work. Not sure how small you can get them either. A good hardware store will have them.
Whatever you use, if you wheel the truck and plan on keeping it, I say seal the hole up good. If water gets into that back reinforced area, I guarantee the top will split at the seams eventually, from rust.