MoMoris,
The airtabs inventor actually tested them in a wind tunnel on a hatchback. He showed a 4% reduction in drag. This however was done by a professional and not an amateur.
they have some of the results in a pdf but no photos of the placement
http://www.airtab.com/MicrosoftWordHondaWTReportV6.pdf
They can work. You may need a wind tunnel to get it right however.