Im going to make a 20" x 3' x 4' grow box. A 400 watt hps. Still not sure if ill use soil or bubble buckets. Inline fans for ventilation. The only thing i think ill have a problem with is the heat from the light.
Cooltube for the light and you'll be set. Edit: Oh and keep the ballast out of the grow area to further reduce heat
Keep the ballast out, and ur better off making your inline fan and OUTline exhaust at the top of the room. If you have some kinda hole at the bottom for intake, the fan I doubt will have more than 150cfm's making it draw the air from the bottom, no need for inline with just an open hole for such a small exhaust. e/ also if you can afford it get a mh conversion bulb.. hps suck for veg, I'd rather use cfl's for veg because hps the nodes are so far apart, they stretch the plant in veg.
...waste of valuable space, imo. you can pull 12+oz with a 400w but not with 20" width. With 4' of height you'd have to mount your fan and filter outside the box, so that's something to keep in mind. Other than that it looks great.