Not sure +5v will be 'man' enough to spin the fans fast enough to do the job, although obviously bigger fans = more air movement. Due to the lack of m'board connectors available to me to connect fans onto, USB is probably going to have to be the route we take.
There are in actual fact 2 fans in the case, both powered by 4 pin connectors off the board. One is blowing air across the CPU, the other is extracting air out of the case and it is almost a 'tunnel' of air in the way it is arranged. Being Dell however, I doubt the connectors are 'standard' (they don't look it).
This http://www.book4e.cn/xie/DSC02848.JPG is what the inside of the PC case looks like, although in this pic, the CPU heatsink sits between the 2 fans as it has been removed to get to the CPU. The HDD goes where the heatsink is shown.
Use the 5v to switch a relay. Use the relay to control a pair of low voltage fans fed from a 12v wall-wart. Or if you can build, or get someone to build, a mains relay to switch the fan or wall-wart.
Fans could be as big as you want!
230V Main Axial Fans : 12V / Mains Fans : Maplin
You'd probably need to get it PAT tested.
Remove the door to the locker and replace it with a couple of removable bars - pref across the switches etc - so no one can get in to muck it up but the server would be cooler
I fear that if the locker door were removed, and the innards protected from 'fingers', we would soon find various 'deposits' left for us, i.e. banana skins, apple cores, crisp packets and half eaten sarnies.
How about one of these water cooled jobs? :)
What about getting a cheap mini fridge and ripping it apart and building it into the cab