I've found nothing that works well and isn't expensive. Nothing at Home Depot is meant to be wet all the time, and I'm not sure of their level of toxicity in a closed chamber with heat and humidity 24/7. I once used non-toxic boat paint and that worked, but it was $90 for the primer and $130 for the paint, and then it didn't hold up to three growing sulcatas where they were rubbing on it. I used Pond Shield on a couple of recent night boxes, and that was okay, but you have to mix it and use it just right and its also $100 for a tiny little amount. I found it difficult to apply and mix correctly. Coverage was okay on the first one, but it doesn't look so great on the second box.
I have two tort tables sealed with Flex Seal. The product works great. I allowed it to set for a couple days, then just added my substrate. The smell is totally gone, it holds water and causes no discomfort for the tortoises kept in there.