When I was a kid, my uncle had a caiman enclosure set up that held a few caiman that were 5-6ft ... this enclosure was nothing pretty, it was just a basic box made out of plywood with a 2x4 frame. It was siliconed and water sealed with thomsons water sealer I believe, amazingly it worked quit well.

Thats might be something to look into

or same basic wood idea but after its built, lay a fiberglass coating inside.

I remember seeing some website online that showed how to build huge fish tanks ... they were 3 sides (back, righ and left) made out of wood and the front was thick plexi ... the builder siliconed, water sealed and then did a coating inside with some type of fiberglass or epoxy

There are ways to build something out of wood ... check into this
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Great Lakes Zoological