It would have to be high enough that there was room for the air to flow through the upper section of the fins, into the box around the area of the flat spot, and then back around the lower section of the fins. From memory I seem to remember 2 1/4 inches high, but don't trust my memory. The next time I'm at the hanger I'll recheck my sketch. I'm building the box from think stock first to figure out how to cut and bend it before I go to metal.

Paul Bricker
N63PB