I used polyurethane varnish on mine, too.
BUT people who make enclosures for a living seem to use some form of epoxy injection resin intended to be used to repair cracks in concrete, to cover the inside of the enclosure/vivarium. Should make the surface close to bulletproof.
The brand they use around here is Nanten IHS or IHP, I forget which is which. It's apparently safe to use in even in aquariums. I'd imagine that it would make just about any paint safe to use.