I have too come to this issue as I am designing my setup out of an old corner TV stand.

I plan on leaving 8" to 14" of space under the where the floor of the enclosure will start. I will make braces underneath, then use a piece of acrylic on the top of the brace with the heating pad under the acrylic. It will double as a storage cubby and will have a finished piece of something covering it. While you're at it, you can also make some short containment walls around the floor, that will sit flush against your walls. Then all you have to do is slide it out to clean it!

Basically, a shelf with a heating pad and a flat piece of acrylic on top. I'm also going to line and seal the wooden walls with acrylic, which should make cleaning, humidity, and mold control much easier!