You might want to investigate the dive brake designs used in dive bombers during WWII. They were not all flat panels, many had holes in them. An obstruction to air flow that is not solid can have more drag than a solid panel due to the airflow through the holes.
The cruciform parachute also has more drag than a solid chute of the same size.
http://en.wikipedia.org/wiki/File:Dauntless_bomb_drop.jpgYou could design the air brakes as a perforated panel that slides out of a slot in the rear of the body. It only has to be large enough to cause a major separation in the air flow and "spoil" the aerodynamic shape.
If you have ever stuck your hand out the window at 70 mph, you know that even a small flat surface can develop a surprising amount of drag. Since drag goes up at the square of the speed, a human hand sized object would have 4x the drag at 140 mph as it does at 70, and 16x the drag at 280 mph as it does at 70 mph.
The German Stuka Dive bomber used narrow slats on the outboard section of the wings as dive brakes, that rotated 90 degrees into the air stream under the leading edge of the wings. You can see them just outboard of the main landing gear in these photos.
http://www.wwiivehicles.com/germany/aircraft/dive-bomber/junkers-ju-87-stuka/junkers-ju-87-stuka-18.jpghttp://www.wwiivehicles.com/germany/aircraft/dive-bomber/junkers-ju-87-stuka/junkers-ju-87-stuka-captured-01.jpgAll you would need to do would be a flush mounted strip about 3-4 inches wide and a couple feet long that would rotate 90 degrees (along its short dimension) to stand perpendicular to the body and spoil the air flow creating a very large high drag wake behind the car. This sort of design would also tend to inhibit spins. If the body tried to spin the slat facing the direction of the spin would create high a pressure zone ahead of it on the side of the body, and the slat on the side of the car facing away from the spin would become less effective and dump the high pressure zone ahead of it on its side of the body. This would create an aerodynamic force that would try to return the car to a stable direction of travel with both air brakes equally exposed to the air flow.
Larry