Although I seldom use it for bread proofing, I have what is probably a
perfect one which I built to use for making my own yogurt.
I took an old, non functioning microwave oven, removed the heavy electrical
stuff and rewired the light bulb so that it was connected to the power
cord. I bought a thermostat intended to be used for a reptile enclosure
(got it on ebay), drilled a hole through the back of the oven cabinet so
that the probe could be inserted inside and plugged everything together.
The thermostat will control the temperature up to about 118 F. and
certainly in the range for bread proofing.
If you want to find an old microwave, try a junk dealer. No one wants one
for repair or normal use. If there is anything that I failed to mention
here, email me and I will respond.
Jerry Ulett