Closet has 1 exterior wall and 1 wall shared with garage. That probably explains the temperature differences. Cheap cookie cutter house probably has the minimum R value to meet code. It gets hot in the summer and cold in winter. I could probably just remove the door and alleviate the issue. But the point of doing the rack in the closet is to keep the kids OFF! Removing the door cancels that out.

I have ordered the ceiling diffusers already. I just have to wait until the T6 is in stock now. They say 12/29 in their site. Anyone know if another low voltage 6" duct fan? :)

Have you cut into the wall shared by the garage? I would advise against that- it needs to maintain the local fire rating.