I know a person that built a cooler off his garage using a large air conditioner. I know, because I helped him build the thing. We build the cooler part with R-30 in the walls and ceiling. Then he mounted a 15,000 BTU air conditioner in one outside wall. On the ceil is a large rail that leads to the main door of the thing. The door was also special built to be extra thick and insulated.

He guides bear hunters in September and it can get a little warm in Wisconsin even then, so he hangs the bear off the ceil rail which you can almost back right up to. They have a scale there also, and a cement floor that he can hose down. He said you put that bear in that room and crank the air conditioner on, and it will cool the room down plenty to keep the animals. I never actually asked him how cold it got in there, but the idea did seem good at the time.

Just in case you have a large air conditioner.