Neil I own a fiberglass car body so from experience I can give you a few good reasons why to stick with epoxy, but Jim or others probably now a few things I don't. Epoxy is stronger then other types of resin used with fiberglass and in an application like car bodies where structural integrity is important I would stick with Epoxy.
Here is what can happen to fiberglass given enough time, weather and stress.
If your concerned about biodegradability of the composite skin a plant based plastic/composite would be better, although not as simple in a backyard application.
Henry Ford was working on building cars with this method almost a 100 years ago.