i've seen a thread on here where someone took apart a mini fridge and used the coolant part of the fridge to keep their res cool.. looked like it worked pretty killer.. it would take a lil work on your end, but not all that much if your somewhat handy...
you could also just purchase a chiller made for this purpose, but a nice sized one runs about five bills or better, where you could probably pick up a used mini fridge for fairly cheap, and diy one for way less money..