I am doing like they do shrimp boats here in Louisiana. They build a wooden house on a fiberglass hull, then coat it in fiberglass, then paint it.
I'm gonna build the box on mine, wrap it in fiberglass, then cover that with Durabak bed liner. You have to put some kind of coating on the raw fiberglass. Marine paint or something like that would work.
You could just build a wooden frame and stretch out cloth and then staple it to the wood, then start laying up fiberglass on that. It's the same principle they use when building speaker boxes. Works really well and really strong. Just gonna take a lot of layers to make it strong.
Example:
http://www.youtube.com/watch?v=G8cz_5eP81U