simple. wrap pipe around a keg
put keg in water tub.
put salt water tub in fridge
drill third whole in side for tap to come through.
mount tap on fridge.
set fridge WAYYYYY COLLLDDDDD.
salt will allow water to become way cold.
if you start with ice it will be mega cold.
heck you could even use a styrofoam box for the fridge.
ice+salt water = very frigin cold water!!!
just like making ice cream
here is a horrible representation.