You may want to have a look at the roof design of the Renault 16 built during the late 1960's and early 1970's.
This five door hatchback used ribs on the roof forming "sails" toward the rear where the hatch was fitted.
Directional stability was superb and even at 100 MPH cross winds had virtually no effect on the stability of the car.
This link show the panels well:
http://www.renaultclassiccarclub.com/16TSnew012.jpg
Just as a side issue the Renault 16 has a number of aerodynamic features which are well worth a look.
Cheers , Pete.
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