How does/can a fan fall off?

In my search around possibilities, I did come across this which doesn't seem badly priced:

Draper Expert 32mm 36mm Thermo Viscous Fan Nut Spanner - eBay, Hand Tools, Garage Equipment Tools, Vehicle Parts Accessories. (end time 17-Aug-10 05:58:11 AEST)

However, given it's the sort of item I'll probably (hopefully) only ever use once, I'm going to see if I can fashion something to 'do the job' out of a bit of decent thickness flat steel, just to nudge it on a bit more securely.