Most computer stores will have cheap little fans if you want to install one. I'm not sure where to wire it up to though.
It is a neat combo to install (inside the case) a duratrax 7 amp power supply with it's own cooling fan. It really makes a nice charger out of the dig pir because it can then charge 7-8 cells at 5 amps on AC power and is all contained in the original case and the fan kicks on only when needed. If wired properly it can also be used as a "power supply" for other chargers, etc.
I've done a few of them and really makes a "charger" out of it.