I would recommend not making the entire top screen. That is the best way to lose all of your humidity. Instead, I would put vents like these...(http://www.cagesbydesign.com/c-51-ad...age-vents.aspx) in the sides and back. You will have better humidity and temperature control.

I would personally put the heating pad inside of the enclosure on the plywood bottom and then cover the whole bottom of the cage with ceramic tiles. They conduct heat well and they will be easy to lift out of the enclosure to clean and sanitize. Obviously you should not adhere them down or grout them...lol.