Well my chiller originally had a somewhat different purpose, and a different control method (no thermostat), which adapts much easier to my plans. It was actually designed to build up an ice-bank over the coils, and it has a sensor that can detect how thick the layer of ice is. When the ice gets to be about 1.5" thick the compressor shuts off, and when the ice is back to to about .5" the compressor turns back on. The thing was originally a Pepsi fountain-machine if anyone else decides to try and get/mod one.
As for a thermostat-based controller, I'm not sure how to go about modding it to work in a similar way. Probably easiest to find a location somewhere in your system that has a temp similar to what the inside of the freezer used to run at, and just keep the original thermostat. On the other hand, the wires coming out of the thermostat to the compressor should be as simple as an on-off switch, so if you aren't worried about variable control it should be easy to build a circuit that turns on (likely closes the wires) when more cooling is needed. I don't know enough about electronics to build one from scratch (I'd get the wrong resistors and stuff probably, or miss some important connection), but I can give a basic idea and there are people around here that can fill in the details for a simple one:
Get a thermistor with a waterproof end, that goes into the coolant (preferably near where pump inlet will be), run a constant voltage into it, and have a comparor on the other side. The other input of the comparor will be the same constant voltage running through a resistor (I don't know what size though). The thermistors resistance changes with it's temp so whenever the thermistor-side voltage is higher the compressor needs to be on (or perhaps the other way around, again get better details from someone else). Anyways - the output of the comparor you can just hook to a relay or something that closes the circuit that the thermistor was originally connected to - that should do it.
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