This is right from the can of stuff I used -- Minwax Helsman Spar Urethane Indoor/Oudoor.

I used one coat of a wood stain, and then put on about 4 coats of the urethane. I've been using the enclosure for almost a year, with zero health issues.

It does smell strong the first few days, and I actually let it sit outside in the sun for a few days after I sealed it. I think I waited about two weeks before adding my snake.

The stuff is relatively expensive, but I haven't had any issues with warping or anything, and I mist the enclosure daily.