If I had to do it, I'd go with radiant heat panels (I like pro-products, but there are a ton out there now).

I think any kind of belly heat/heat tape and there would be a safety issue. Under the cage it would be compressed which is dangerous and inside the cage there is no way to keep the snake off of it. And I just hate heating with bulbs and CHE's (personal preference)

Radiant heat panels are cool because they actually heat the surface of items in the cage (floor, hide box, etc) causing a sort of "faux" belly heat. I've used them before in neodesha and homemade wooden cages and they worked great.

Hope this helps and good luck!

-adam