I'm looking for the best suitable timber based material to sit inside my vivarium so that I can sit the heat at directly below which will enable me to use a heatmat as a sole source of heat for one of my BP's. The vivarium is an oak formica board setup, which is 18mm thick therefore the heatmat under the vivarium doesn't transfer enough heat. I'm looking to cut a sheet and apply it inside the vivarium, with heatmat in between. I'm thinking some 4mm plywood but just wanted to get your opinions on how it would stand up to the constant heat and moistures from humidity and toilet. Any ideas and advice would be appreciated, ideally I don't want to have an overhead light and I believe the belly heat is best.

Thank you all!