Whether or not a tub is in a rack is irrelevant. I'm sure there are alternatives but the number one way to set up a tub or rack system properly with only belly heat, is to heat the room. My room is kept around 82 degrees so all I need to do is regulate the hot spot.
I think you could also have two uths. Each covering half of the tub. And run those to two separate thermostats.
One uth on the warm end in a cool room will not warm up the cool end enough.