Good point!
Which is why I mention a single tub rack, it can be enclosed for better heat.
I like ultratherms for some things, but since it has to run almost full blast to get the desired temp(90+ hot spot).
And the ungodly price for it versus a piece of flexwatt, I would never use one under a tub in a rack, but that's just me, I hate wasting money for no reason.