Geo Metropolis -
I used 4 small zipties on a plastic wheelcover too :
http://ecomodder.com/forum/showthrea...-diy-9158.html
I used 4 zipties because my wheel has a design pattern that has 4x4 holes. However, my cover thickness was maybe 1/16", so the zipties were not holding much weight.
My design has a slight conical shape that made the edge slant *into* the edge of the wheel. For you I am guessing that the air may be getting in and "pulling" on the edge of the wheelcover. Can you have someone pace you in another car and watch their behavior?
CarloSW2