I suppose you could drill a tiny hole in the humidor and glue it afterwords....powering the fan I use a radio shack plug in deal (normal 3 plug wall outlet with selectable voltage) on my CPU fanned router
You would have to check the volts it takes to power whatever fan you would use, which isn't hard. I'd get an interchangable power source though.
Looks like it is taking 12V to power my CPU fan, it's a beast though
For the record I never did this to a humidor, but I did power a CPU fan and mount it on my router.