If you're not afraid to open up the case, there is a 12v header on the main board. Two pins at the back are BOTH 12v, connecting a fan to them will do nothing but connecting it to one and the chassis will make it spin. Amazing but true.

I put a small fan in mine and found it cooled things really well but was too noisy and not having the electronics will to put in a thermostat or resistor to turn it down a bit, took it out.

are you getting a lot of overheating?