How to wire electric fans to stay on for a few minutes after engine is shut off?

[nikwho]

Hello all,
I've currently got my electric fans wired to a relay with the trigger wire hooked to a constant 12v, switched by a 180°temp switch, so that the fans continue to run after I shut down. However, my radiator doesn't have a bung for the temp gauge, so the fans run until the coolant in the intake manifold is under 180°, which takes a fair bit of time. I don't want to end up with an issue that drains my battery, so I'm wondering how to wire this thing so that it runs for a set amount of time after I shut down? Is there anything that I can install that gets 12v and shuts piwe off after a set amount of time after it stops receiving power from the ignition?

Thanks,
Nik