The cheapest commercially available Differential temperature controller I can find is $70.
I think a kit for $15-20 would be a valuable addition to Velleman Kits.
They would be similar to these:
solorb.com/elect/solarcirc/diftemp2/
freeinfosociety.com/electron … hp?id=2127
I would prefer however a form “C” relay output and use more standard NTC 10K thermistors.
I have bought the components to build the first one, but without a pre-etched board I’m finding it too difficult.