I think mark Pascoe built one into the seat of an SR something, but all you need is a refrigeration unit (someone makes these for boats caravans etc) have a look at the yank ooooops american sites like
www.westmarine.com - it's all 12 volts so it all works the same over here.
Bear in mind they use a lot of juice so you might want to wire it up to the ignition side of the system otherwise you'll have flat batteries!
Good luck and if you get it working can you cool down a couple of cans of draught Guiness for me?