well DUH! why not use a flexible magnetic vinyl strip? this way you could easily adjust your location until you found a best spot.
either that or smear some oil on top of your car and then drive around at high speed and find the "bubble" and mount your strip inside of that "bubble"
or i might just make some of these out of plexi:
site can be found here:
http://www.fuelsavers.com.au/
Oh! Be sure to not draft any trucks that happen to have these mounted on the side!
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