Are you thinking of using fiberglass just to form it out then maybe an aluminum roof? The entire thing out of fiberglass?
Either is doable but I promise you what you save in money you'll spend in labor 2x. As long as you're a DIY kinda guy and don't mind spending a lot of time on it!
I don't know of a specific how-to on building a roof but the concepts would be the same as for a fiberglass rack enclosure. Felt, fiberglass & resin, a helper and some time. If I were going to undertake a complete glass roof, I would make it in two pieces and glass them together.
I suppose you could use wood but I can't see that ending up looking very clean.