Welcome aboard!

BP's do very well with just an undertank heater....IF the overall ambient temps in the enclosure remain around 80 degrees. An undertank heater alone will not raise the ambient temps if the room is cold. That is usually when overhead supplemental heat is recommended.

And yes...that overhead heat can dry out the air. A dry towel over the top of the tank probably won't help much. A damp towel (changed daily) might help better. Getting a sheet of clear plastic cut to size, with some corner spaces cut out for ventilation and a hole cut for the lamp...use that to cover the screen...that can help, too.

One more thing...they don't really need (or want) a drop in temps at night. It's important that their overall ambient temps stay around 80F. Constant fluctuations in temps can put them off their food. If your heat lamp is a white light, you'll want to switch it out for a red or blue one...or a ceramic heat emitter (which looks like a bulb, but just puts out heat and no light).

OR...make sure the room temps stay warm and just go with an undertank heater....OR switch to a shallow plastic tub, which makes maintaining a proper environment much simpler in the long run.