Wow- minimal stealth requirements.

Kool- makes it easy.

You just need a decent fan mounted near the ceiling of the closet, blowing out thru the door- just make sure you close the door during your dark period.

If you want to lightproof for flowering reasons, pick up a " duct booster" fan @ Lowe's or the depot, and some flexi duct in the same size. Also a piece of heavy plastic ( ie Panda) large enought to cover the whole door.

Hang the fan in the closet. Attach maybe 5' duct.

Tack plastic over door ( tack from the outside, so you can pull it off when you want to go in.). Some of that sticky-backed velcro would work really well.

Cut a hole in the plastic up by the door header, large enough to pass duct thru. Pass duct thru, and tape up . Let the duct hang down at least 45 degrees to prevent light leaks.

Leave the bottom maybe 6" of the plastic free, so that air can get around it. Make sure that it's hitting the floor, though, so you don't get light entering there.

Run the fan 24/7. You might want to put a thermostat/humistat on it when things cool down this winter.