Here's how I decided to handle the cooldown timer on my projector. I already had the parts sitting around but if you don't then you can expect to spend about 8 bucks on the relay and a couple bucks on the wall switch and cover plate. The FET can be sampled free from Fairchild, though any N-channel FET should work if you already have one. I used the relay because I wanted to use my single-pole wall switch, but you could instead use a DPST switch capable of carrying 5A at 120 Vac and eliminate the relay and wire everything directly to the switch. In either case, be very careful when wiring this thing since you don't want your 120 V and 12 V lines mingling!

When the wall switch is turned off, the capacitor C1 will hold the FET channel open until resistor R1 discharges it, which takes a certain amount of time. In this case, the 2200 uF and 43 Kohm combo yields a 3 minute 5 second discharge time, so the fan will run for this amount of time after shutoff. Raising either the cap value or the resistor value will yield a longer cooldown period, if required. Vice versa is also true. I had that cap in front of me at the time but you can use a smaller one (470 uF, for example) and compensate by using a larger resistor value (220K or so).