Stacy,
I assume you are fully enclosing the closet somehow, either extending the walls up to the ceiling or putting on a roof. Either way, you will need to somehow ensure the exit air passes by the motor, forcing hot air from the top.
My ceiling is 8' high. I had to build an alcove up between the joists for the motor and to ensure a couple inches of headspace above the motor. The exit from the alcove into the plenem exits from that alcove to ensure the motor is cooled. My folded 2 layer plenem is 5' long each way so 10' long total with a cross section or 110 sq in which does a good job of baffling any noise in the exit air. I believe that Ed's baffle forces the exit air from the top, down the side and out at the bottom. I think this should cool the top air and motor in your case just fine. The air from the filters should easily cool the motor. Before I built the closet, I originally had a small blower that forced cooler air past the motor in the alcove. With the closet, it is probably no longer needed, but is still in place anyway.