You could ghetto that door with duct tape and cardboard or you can just buy a piece of 1/4" or 1/2" ply or any wood as twitch stated up there, buy a set of door hinges and some thick tape of some sort to seal the cracks. Then buy one of those cheap pad lock hinges. Thats all you need for an enclosed air tight box. Now for ventilation,multiply length x width x height = cubic feet. To replace the air inside the internal volume once every two minuits you would start your calculation by cubic feet devided by 2. Example. 3X3X3=27/2 = 13.5cfm. You will need a minimum just for a 3x3x3 space an intake and exhaust vent that is capable of 13.5cfm(dont forget to give some headroom to count for efficiency or turbulent loss) when you add lights, try to double the cfm per every 15 degrees of increase and give even more headroom if your trying to use a carbon filter. You could have more then 1 fan, and just keep only one on while the lights are off if its too cold where you live. Remember, intake low and exhaust high on the other side. For lights, well pretty easy. Paint the entire inside white or fabricate the walls with mylar, then use cfl's or LED's. You will probably have to bring your cubic foot formula to cubic foot devided by 1 if you want yo use some type of HID lighting.