I've been chatting with Teri on this subject and am of the opinion that for the kind of riding I normally do ANY tendency to veer from a fluke cross wind would be very dangerous. In heavy highway traffic big rigs generate very complicated gusts . Having a sudden lift applied in that situation would not be good.
I've come to the conclusion that a straight sided fairing ( no convex surface) but with a flat taper to the rear might solve this problem but at the cost of aero efficiency.The front would have a wedge shape vaguely reminiscent of this photo posted earlier
http://bikeweb.com/files/images/royce-578.preview.jpg .
Gentle rounding to reduce drag but no convex surfaces . As can be seen in the photo, the flat sided shape lends itself quite well to closing "doors" on the sides but probably with foot opening down low.