I started with two 4x8 sheet of urethane insulation. I cut it to make a structure 8' tall, 2' wide adn 3' deep, with floor and a ceiling that I could flip open the front half. I cut out the bottom of the door for a small window air conditioner. Everything is duct taped together so I can disassemble if storage.
The air conditioner would normally not get cold enough, so I took out the thermostat and reset it. The thermostat is normally simply a bimetalic material that makes contact with a connection when the temperature goes up. The dial screws in and out the contact to adjust the temperature that it comes on at. There is a calibration screw on it that I screwed in until it would go down to 40 degrees at the coldest setting. I don't have it in the picture, but I have a cheap indoor/outdoor thermometer taped to the door with the wire probe going in and hanging in the chest cavity.
As described in my other thread,
this is the biggest deer I've ever gotten. I had to clip his legs a bit to get him to fit!