My advice is not to accept the easiest solution - polyurethane - until you've looked at other possibilities. Poly is fine for dressers and handrails, but IMO sucks for boa cages where 6ft boas take a dump that is huge and expel lots of liquid too. There has got to be a better way.
Check out epoxy paints. I suggest going to a paint store that has epoxy paints and find out what you can. Even cheap-@ss Home Depot has one-part epoxy paint that might work. At least check it out before going with poly.
Also check out epoxy resin. Very messy but dries into a hard clear coating.
Like I said, at least investigate other options. BTW, this is why alot of people build cages from melamine: no finishing necessary other than sealing the joints with silicon caulk. Melamine is heavy but inexpensive. Just some thoughts for you. Don't take it as gospel (and don't think poly is your only option!)
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Mark 