well. few pics from from my closet air system.
fresh air circulation is very important, and even u keep opening the door to your grow room, its not gonna be enough as ssoon its gonna get more bushy...
I have one fan to suck air IN (just regular fan with modified cardboard (!) box, that makes 4 separate air streams). fan is located in the bottom of grow cabinet.
fan for sucking air OUT is window fan. as u can see I am using pipes u can find in ducting/ventilation section if hardware store... fan is located on the top of grow cabinet.
blue arrows marks fresh air. red arrows marks used one...
your grow box is pretty small. U need just fan IN I guess... if I was u I would just take one of your grow box nice doors away and guess what I would put in???
yessir. u right . an ugly peace of cardboard. ha ha. I would make a bigger hole in it an I would connect fan. I would leave few small holes in the top for air out. during flowering u can cover small ones with carbon filter for smell reduction....