Fiberglass will be fine as long as you allow a minimum one month cure time. Also, make sure it is properly reinforced with an integral frame work. If you do not, it will warp under its own weight over time in the presence of water and warm temperatures. To be a suitable long term construction, you will need a minimum thickness of at least .25 inches.

Personally, I find fiberglass based constructions suitable only for extremely large enclosures due to both cost involved and the instability of the medium in humid environments.

If you do take a gander at vivaria forums, be very careful with the advice you get there. Very few of the builds pictured there are built with amphibians in mind. Even scarier, many of the builds that are built for amphibians are made with unsuitable materials or are improper habitats for the health of the animal. It is a great place for ideas, but the quality of information there is pretty touch and go. (I myself no longer go there anymore for that reason.) We have many skilled vivarists here (and over at caudata.org) that probably know more about vivaria for amphibians than anywhere else on the web. Dendroboard is another forum you should check out. while those folks have dart frogs in mind, they have some expert vivarists there too.