That sounds like a good idea, unfortunately my custom built hushbox is on the floor. I was hesitant to cut my celing, and or floor. So I made a box using a glass end table as the frame and then "building" the box around it. It is lined with sound absorbtion material, and cuts down on the noise significantly. Currently I am using a rectangular window fan purchased at Wal-Mart to push the air from bellow and out a vent cut in the top/back of the box. The problem is this fan is nosier than the projector fan, and it's the quitest "real" fan I could find.
The box has a hinged front with a window of edmund scientific glass, and is covered in black velvet. I then put one of the glass pieces back on top of it. It actaully looks pretty good. It is not completely sealed with openings on the bottom for cool air to enter. I just need some quiet fans to help move the hot air on the top level out the back. Hence the need for smaller/quieter fans.