Cage sealant?

I got a 7 foot red tail boa at the last reptile show and I need to build him a cage. He's in a really heavy duty bin right now with a heat pad that has the proper temps but is too small for a permanent cage.

A few years ago my family built 3 5 1/2'x2'x2' wooden bins, and we still have 1 left. I already cut a hole in the front for a door, and I need to waterproof the inside of it. What is the safest and most cost-efficient way to waterproof it?

I'm using minwax superior durability. And then I'll be laying tile to cover heat tape and to add in sealing. It's not the cheapest but it's not to expensive. And the minwax is a polyurethane an you need to let cage air out before you put your snake in it. It has a strong smell for a little while.