If your unit is heated electrically, I am assuming the water level in the
reservoir is maintained by a float valve system of some sort.
If you check with a refrigeration repair tech you will see an assortment of
cheap plastic ice machine water float valves that should be able to be
adapted to your proof box. I doubt the temperature in the proof box would be
damaging to the plastic. If the heater actually boils the water, then you
will need to locate a metal type valve.