I have a similar situation, my whole building is made of block, I am the super and wanna start in my closet instead of a grow box in the closet. There are no holes or anything and it has those shitty hanging sliding doors that you see in apartments all the time. What I plan to do is just inside the door is put one of those room divider tarps with a zipper in it just inside the sliding doors so it still looks like a closet. There is another small shoe closet next to it so I plan to make a 4" hole in the drywall ( easy to repair with drywall, tape and mud, sand then paint, will look like it was never there) going through to the closet nextdoor with a carbon filter hooked up so it doesnt smell, and leave the closet next to it open all the time so it sucks all the hot air out, especially if you have a cool tube this would work, also i find just basic air circulation helps some, "blowing the air around. so adding an oscillating fan (which you should have anyways to simulate wind) will blow the air around.. you can have a grow without a fan unless its outdoors, the lights required most always produce large amounts of heat. I would recommend purchasing a small exhaust fan to blow over yous ballast and bulb to pull the heat off it and another to pull the air from the room, but it must vent to out of the room or your just moving the hot air around...