One thing you might try is an aquarium grade sealant sold by aquatic ecosystems inc.
HTML:
http://www.aquaticeco.com/subcategories/3048/Sweetwater-Epoxy-Paints-1-Gallon
. This stuff is kind of pricey, but it goes a long way and is used to fabricate aquaruims out of plywood. I have used it to seal a large enclosure that I am building, and it has worked out pretty well so far.
Also, there is a gentleman who posts on another forum who has built several enclosures using another product.
"Siliconing the corners will not be required if you're using the epoxy after construction. Just goop a little in the corners.
If you're applying the epoxy resin before assembling the enclosure then I would silicone the corners just as a precaution.
Make sure you are using a safe epoxy such as the WEST SYSTEM Epoxy Note that there are many marine epoxies that are not safe to use.
Any child-safe latex paints would be a good choice to use. Rustoleum does have a good line of colours in their Painters Choice line."
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He goes by the name KRIMINAAL, is an administrator on the viv construction forum, and can be found at the herp center network <www.herpcenter.com>