Go to a home depot and find some small vents, like for under soffits on your house, you need to get some fresh air flow in there and keep the air moving. what you could do then is put a small vent on the back side of your stealth, if it say against a wall, then put another small vent up top, on the back side, if possible. But you need to get fresh air moving through there for them to grow. How big of an area are you working with. Pics would be helpful but i can do what i can to give ya a few ideas
How many plants are you growing?