I built my grow into a 2'x8'x7' closet with a shelf 20" form the top extending the length. I first planned on using t5ho as the lighting for all areas. I got a small can filter and a 140cfm fan and cut a hole in the divider shelf at the top of the closet. Put in the fan and filter, and cleaned everything with bleach.
I repainted with flat white paint, and constructed walls out of R-Matte Plus-3 3/4" thick foam board. Everything is taped with flex -foil tape, and this is covered in wider foil tape for light tightness.
I use a standard household a/c register to hold a 12"x12" pleated filter. I use the best 3m Filtrette available, and everything has a gasket on it. This flows into a light trap box, 23"x24"x20", then down 2 6" flex ducts to the far bottom corners of the room.
I cut out a door, and made a frame to go behind the cut out, This allows me to glue/tape frame in place, and put a gasket on it. Then, using flex-foil tape, and considerable care, I ran tape the length of the edges on the door, using pressure to slightly compress the leading edge. This basicaly beveled it alolowing easy application/removal. I also did this to the cut out so it is a smooth fit. When the fan is on, It pulls, it into the frame, creating a light tight seal against the gasket