I wouldn't ever use "adhesives" either. What if one became unstuck and sailed through the windshield of the car behind you? That thought was enough to scare me away from "stickies".
My fiberglass top came reinforced with 1" hollow steel ribs. They are incredibly strong, almost like roll bars. So I used them, drilling holes through my fiberglass top to run 10-inch long headless bolts through. Below, I used pipe clamps (the kind with a screw on it, to secure each bolt to a rib inside the van. I secured the top ends of each bolt to the side of a solar panel with a stip of steel plumber's tape, which is bolted to the solar panel frame with nuts that have the plastic insert to prevent them from turning. So if someone wanted to steal a solar panel, they would require two sets of nut drivers, one for the hex bold head and one for the hex nut. The solar panel is prevented from slipping up and down the 10-inch bold by a nut screwed onto it above the steel tape, and another nut just under the steel tape, so the panel can't slip up or down. Those two nuts can't be turned because one flat side of each is pressed firmly against the solar panel by the steel tape being stretched very tightly across the 10-inch bold.
To prevent water leaks through the holes I drilled, there is a washer below and a washer above each hole, the two washers held tightly together pinching the fiberglass between them with a nut against each washer. And of course, I filled the hole with silicone sealer before tightening things down. It's worked great for me for nearly three years now. But the remaining question is, does your fiberglass top have strong rubs that can support solar panels?
