Lets make one thing perfectly clear, heat pads or UTH are to be placed on the sides.
and to be controlled by a rheostat or some type of controller.

Even if the enclosure doesn't have burrowing inhabitants.
Because if the glass gets hot even with a UTH and water happens to come in contact
it can shatter the glass or at very least crack it.

The only time I have ever used a UTH under a enclosure is when it is mounted on a ceramic tile
and the tile is placed under the enclosure and not in direct contact with the bottom class
to give a soft indirect heat.