The cheap DIY Tower cooler is as follows: Take a computer fan, connect a funnel on the output side of the fan. Connect a hose to the funnel (small enough hose to fit up the tower), push the hose up the tower so that the end of the hose is near the top of the tower. Hook it up to a AC to DC adapter and your done.
The hose should be long enough so that the fan is near the bottom without touching. The coolest air sits at the bottom and hot air moves to the top of the kegorator. This project will also help move around the cold air to stabilize the overall temperature. Also, you don't want the fan to be too close, or touch, the bottom of the kegorator. If you have a keg leak, your fan will fall victim to shorting out due to liquids in the circuitry.
The fan can come from an old computer your not using, and look for an unused AC/DC adapter. I believe a DC 12 volt will work. Someone correct me if I'm wrong.
Cheers