This really won't be practical for most users of belly heat.
The proper way to use a UTH is to adhere it to the bottom of the enclosure, either using the permanent sticky that it comes with or using aluminum foil tape.
You must control the UTH with a thermostat to prevent burns. Any kind of re-usable substrate will get dirty to the point where it is no longer cleanable. Your best bet is a UTH on a thermostat with disposable substrate.