How warm you keep your room is going to dictate how you are going to have to heat the tub. If your room gets down into the upper 60s in winter, your tub's ambient is going to get too far below a safe range. The under tank heater is not really going to do anything for the ambient temps in the tub. Can you use a CHE or heat lamp? Yes. Should you? No. The problem being is the tub has a plastic lid and over head heat emitters can get hot enough to soften or warp a plastic lid. Not recommended to do as it does present a hazard. Others use them but personally I can't recommend it.
Some creative solutions have been to get a second heat mat and thermostat and use that as a makeshift radiant heat panel. The only issue is you would need that outside the lid which creates wiring challenges as you need to have the heat pad wire and probe attached to the lid which you open to clean and feed the snake. Some however put a heat mat insead on the side of the tub which might work depending on the size of the tub.
If you room stays around 75+ degrees year round, you don't have much to worry about.
Just remember this one hard and fast rule... under no circumstance for whatever reason should you ever place any tape or sticky pad/velcro/dial inside a snake's enclosure. If something need to be permanent, you glue it down, never tape it.
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