Creative and imaginative use of common alternatives. Throwing this out there. Why does it have to be fiberglass? Yes I know why and if you can pick one up cheap that fits, great. The way to go. But if you can't find one, and removal and reinstall....
Here's a thought. Make one out of plywood and coat with poor man's fiberglass or just roof elastomeric coating. Did this on my minivan three years ago for storage and still going strong. The curves on a roof make it tricky but scribe to match. Alternative to waiting to find a good one.
Also, for a pickup topper with blunt nose, make a front deflector or storage section with an a aerodynamic angled front. Plywood, sheet metal, etc... Saw a big dually pickup with a big rig deflector on the roof for towing his covered trailer.