Since you are thinking about a boattail, take a look at
http://ecomodder.com/forum/showthrea...tml#post481536. He's shedding parts on the highway. BamZipPow has also run aground lately. I'm not a big fan of coroplast and duct tape.
Look for a build quality that would pass tech inspection at a race track. I just got a good part for an airdam on my car. It's the 'towel rack' bumper overrider from the American spec 67-down Beetle front bumper (cut down), and it will hang upside-down under the later bumper with tabs on the bumper brackets. I've got steel sheet for the diverter.
My goal is to get Prius-like mpg, starting with a 0.40 Cd. But first it's off to the Woodburn Drag Strip for the NW Bugrun on Saturday. Oops, that's today.