This is a very useful device that you can use in many different situations. I primarily use this for regulating a cold frames temperature and for my seed starting tent. This uses a simple temperature switch wired to a computer fan.
Step 1: The Switch
W1209 DC 12V -50 to +110 Temperature Control Switch Thermostat Thermometer that I found on Amazon is the core of this little project. They are 6-8 bucks depending on the range that you want for temperature control.
If you follow the wireing diagram that they send with the sensor you can hook up a simple computer fan ground and 12V source.
Remmeber I am not an electrician and you wire this up at your own risk.
I used an old 12V AC adapter from an electronic device to provide power. Just make sure your volts match up on the adapter and their diagram.
Step 2: Attachment
I used double sided tape to fasten the foil so that it fit snug in my seed starting tent. Then you drop the temp probe down wherever you are regulating temperature and set to your desired temperature. When it gets to hot the fan turns on and pulls hot air from the top out to cool off the items inside.
I printed a box on my 3-D printer to hold the electronics and protect it from the elements with a little piece of weather stripping. You could also hook this up to a solar panel for remote areas like your garden cold frames.