Having a walk in butcher shop would be nice... albeit expensive.

They make something called a "cool bot" It works with a window shaker a/c unit and allows you to maintain a lower temp and must have some kind of a coil heater with it too to keep the thing from freezing up. I'm sure it greatly shortens the life of the a/c unit but they do work well. A freind of mine has this setup in his walk in cooler at his butcher shop. He's into his second yr with the thing and its still going good.
Also look around, someone is always tearing out or replacing walk in coolers or freezers, they are just panels that lock together. Thats what my freind did, found a bunch of the panels and put the window unit with the cool bot in it. Works great.

There is a thread on AT where I guy turned his shed into a meat locked doing this exact set up, let me see if I can find it and post a link.