I use a closet that is divided vertically by a fixed shelf. I have an exhaust fan in the upper bloom section and passive intake at the bottom of the lower veg section. A passive vent allows air to move from veg to bloom, with black plastic stapled to the shelf and back wall above and below the hole to block light (basically the same idea as using PVC elbows painted black inside - the result is two triangular 'tubes' - one above the other and connected at one end by the hole in the shelf if you follow the idea) The shelf hole is located all the way at one side, and the openings in the plastic where air enters/exits are about midway across the back wall.

The shelf is flush with the front of the closet, so I have a 6 mil black plastic curtain over the front of each section with velcro around the perimeter to create a seal. Access to either section is by pealing back these light curtains.

The closet I'm using used to have three drawers down below and two vertical 1/2 width doors up above. The upper section had movable shelves - one has been left in place as a light support which I can raise or lower as needed.