If u want to keep it stealth u are going to have to cut a hole in the ceiling and vent it right thru there into the wall cavity... The only other option is to vent out the closet door to a window. The problem here comes after a few grows. The moisture will build up inside the wall and start to rot from the inside out. A good carbon filter is crucial and a if u can rig a dehumidifier into your exhaust line u never would have to worry about moisture in the wall. PM me for an idea I have for an inline dehumidifier.