One can use a water bath in which he puts the bottles until the last moment, and then pour the chemicals in the processing tanks when the film has been loaded onto the spiral. JOBO used to produce such a water bath called the TBE-2. It is now difficult to find, and it is often simpler to build a system with a thermostat and a kitchen appliance heating element. Also one can build a system to keep all the tanks at the right temperature during the processing, that can be useful for color and in particular for E6.
It is easy to find temperature controller for small amounts of money. Check that the maximum current (Amps) of the output relay is compatible with the power (in Watts) of the heating element you would be using, knowing that: P [Watts] = U [Volts] x I [Amps].
For those of you who like to play it fancy, the professional JUMO eTRON T100 comes with an LCD display for the live temperature value and setpoint that doesn’t emit light in the dark . It combines with a PT100 or PT1000 sensor and can either be ordered as a straightforward thermostat with configurable hysteresis or as a PID regulator. It also features as an option an input for a liquid level sensor (to avoid burning the heating element if it happens to heat in the air).
Keep in mind that water has to be recirculated to obtain an even temperature in the water bath. A simple aquarium pump will do the job.
There are also dedicated device like the TCS-1000, that can heat and recirculate water at the same time.
One can also go for used professionnal lab equipment (as these are quite expensive new).
There is an american brand called Grant for example, or Julabo in Europe.
