Well if you took an old junked fridge with the freezer compartment in the top and carefully removed all the vital components and then built a custom insulated res (make it out of styrofoam) which the evaporator can go into you could make a nice little compact unit that would give sub-0. Alternatively do the same thing with a fridge that has the evaporattor at the back. They are designed to hold the contents of a fridge at 0-5C and will probably give you around the 10-12C mark with plain water.
If you want a "how to" go look
here at the article I wrote on my own system. Box or chest type freezers are a bit complicated to convert because the evaporator is piped all the way around the box