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?