Why don't you just use melamine? Its particle board already coated with a type of plastic. Its still not completely waterproof but close enough. Corn enclosures don't need to be fully waterproofed since they aren't housed in wet conditions. Most treatments will only make wood water-resistant, not actually waterproof. The only method I have stumbled across to truly waterproof enclosures are epoxy resin. You can also use shower liners and the like. For the most part, people generally stick to polyurethanes for wood enclosures