I wonder if it would be possible to heat a small ultrasonic cleaner with one of those little electric submersible coffee warmers. Maybe you could heat the cleaning solution with that, right in the ultrasonic, and then when it is warm, take out the heater and turn on the ultrasonic.
I did this with my first ultrasonic, which was unheated. It worked,
but I never cared for how it sputtered around and had a slight
burning smell. Instead, I usually preheated the liquid in the
microwave or heated a chunk of metal with the torch and put it into
the ultrasonic (usually twice got the solution nicely warm).
Ultimately, I got sick of it and bought a heated ultrasonic (Harbor
Freight for $60). Love it.
Jamie