Cars do have a tendency to float at higher speeds making them more susceptible to cross winds moving them around. Not much you can do but maybe stiffer suspension and lower to the ground? How windy can it be by you?
Aerodynamics wise
Volkswagen XL1: 2013-present (Cd 0.19)
http://arizonaautoscene.com/wp-conte...8__TS30870.jpg
That's a very slippery aero setup. You essentially want less vents and scoops.