If you are building it fairly air tight and have a good exhaust fan, depending on what type of lighting you will use, maybe you could get away with a flap. For example, a gap in the bottom of the door with a flap of mylar hanging down. The suction of the exhaust fan will draw air in, lifting the flap. If the exhaust fan switches off during night cycle, the flap will return making it fairly light proof still. Also a small fan blowing at the plants and the light/s will help strengthen the stems and assist air flow in and then out toward the exhaust. Itā??s probably a dumb idea but I love a good basic setup. Especially if itā??s cheap. Iā??m currently running a single plant setup this way. What are your thoughts on going hydro?