I don't really have much to add to what has already been suggested. I'd just like to point out that you might find it hard to get the heating pad to penetrate the wood well enough to heat. I like the idea of cutting out a piece of glass to fit the bottom of the cage, then raise it up with some wood at the corners. Then you just have to put the heat pad under the glass, and you have an easy surface to take out and clean when it comes time to clean up the cage.

Oh, and for the cage front, I'd go with a sliding glass front, or a glass(plexiglass?) front thats hinged at the bottom so it opens downward.

Looks like a good start so far. Good luck with it.