Building a fridge from a freezer works, but as the freezer has a stronger cooling system, water from the air condenses easily. So it's wet inside, I put a towel on the frezzer-floor. The device is plugged between the mains socket and the freezer.
See also a similar approach by MakeBromyardFeatured: www.instructables.com/id/Home-automation-101-fixi...
Step 1: Find Parts and Assemble the Device
Find an external refrigerator thermostat. It should be air sensing (not evaporator-tube sensing) and it should have a wheel or something to control the desired temperature. Temperature range should include fridge temperature (around 5°C). Also find a housing for the connections, unless there is enough space in the thermostat and other parts.
Assemble them like in the pictures with a plug (1), socket (2), and the thermostat (3). If the thermostat has more than 2 connectors, try one of those possibilities:
* 11 and 12
* 11 and 14
* C and NO
* C and NC (see https://en.wikipedia.org/wiki/Relay#Pole_and_throw what the letters mean)
Step 2: Test
After all boxes are done and closed, plug the device in a socket, plug the freezer in the device and put the capillary sensor in the freezer (in the air). Adjust your freezer to minimal temperature. Adjust the device to minimum temperature. The freezer should operate. If you can, adjust the device to 30°C. It should switch off.