Fiberglass is a neat idea for a vintage rock guard. In the 1950s and up to 1966
, that's what Airstream used. They used an un-gelcoated corrugated fiberglass in either a milky white or a green. Fiberglass is an excellent material for a lot of exterior applications. The only downside to fiberglass is that it is not at all transparent. It is more translucent at best. The other issue you'll need to consider is the structure. You'll need some sort of corrugation (a 3-D pattern) in order to give it strength. But the original 70s guards had a pattern to them that might work. Keep up the great ingenuity. We definitely need more old-school solutions to match the vintage trailers.