I tried to block off by brake ducts using the rubber pipe caps I have seen mentioned by others on this site. Unfortunately the 1.5" and 2.0" caps were both too small, probably because I cut material out of the duct opening. But I found a simple and cheap fix......black foam balls. Soft enough to compress but they expand to stay in place.....hopefully. Will know more after I drive around. Had a hell of a time finding black foam balls that would do the trick......who knew.
http://www.zoro.com/search?q=foam+ball
http://www.zoro.com/search?q=foam+ball