I use Epoxal 3:1 available from Niagara Protective Coatings. It will provide a tough-as-nails, completely waterproof seal for your enclosures, making them safe from whatever you choose to use underneath. It is used for everything from swimming pools, pharmaceutical facilities, croc enclosures, floors, and snakie cages
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It's easy as pie to work with (provided you use it in a well-ventilated place) and actually works out to be cheaper than even the cheapest urethane at $55/gallon ( I didn't even use one gallon on 6 large enclosures, inside and out) - you don't need to be applying coat after coat, one or two works perfectly. Urethanes won't give you a 100% waterproofed seal, or give you any proteciton from any underlying toxicities. It also has to be re-appled every so often (depending on use), as it wears out with time and use. You have to give it many coats for a decent seal on it as well.
EDIT: Epoxy also comes in clear (as in will not even so much as change the hue of the finish, just make it shinier), whereas urethanes often provide an amber hue to wood finishes.