The air tabs borrow heavily from the profile of the NACA duct:
http://en.wikipedia.org/wiki/NACA_duct
Here are some detailed profiles:
http://www.recumbents.com/wisil/nacaduct/naca-duct.htm
Now get yourself some sheets of pvc foam (or equivalent), double faced tape and an Xacto knife (or equivalent) and experiment. Note that the edges of the duct are sharp to precipitate flow separation and create the vortex.
An aero engineer's perspective on air tabs and vortex generators:
1. Vortex generators on aircraft are not 'shark fins'. Those are marketing gimics. VG are generally rectangular. 'Shark fins' act like delta wings which are designed to produce lift. You don't want lift. You want vortices. Hence, you want a very low aspect ratio wing (a rectangle slightly longer than it is high) angled at about 20 degrees to the prevailing wind.
2. [Big] Aircraft generally don't use VG to reduce drag. They use them to maintain controllability by preventing separation in front of control surfaces. You see VG in front of ailerons. You don't see them in front of flaps.
3. Of course, for every rule there is an exception. Some Boeing 737s have shark fin VG to try to reduce the body drag caused by the very short tail cone. But on an aircraft 12 feet in diameter, there may be six VGs. Not every four inches. And this was established on very big, very expensive wind tunnels.
4. Yes gliders are using very small vortex generator strips (mini-air tabs?) to reduce drag. But this may only work on gliders because of the obsessive care to the finish. Aircraft (and cars) that have to work for a living get dirty. And they drive through air that has been driven through by the hundred cars in front of them. The calm air that provides laminar flow probably doesn't exist during most commutes.
5. Vortex generators that stick above the surface add drag. They must be used judiciously. They are only valuable if they reduce more drag than they create. The car that is fuzzy with little shark fin VG has probably only succeeded in increasing its apparent volume. Yeah, yeah, yeah. Sharks have toothlike denticles that improve their movement through the water. If your car is moving through the water, you might want to contact your insurance company. Sharks have had 1/2 billion years to get it right. I don't have that much time.
So, VG produce drag and sometimes provide a net drag reduction. They need to be applied to a specific area to solve a specific problem. And a few being good does not equal more being better.
And yes, I do have some places on my cars where I would like to try them.
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