I am honestly not sure how much air dams actually increase the frontal area. We say not to have them hang lower than the lowest part of your suspension. Basjoos has double side skirts. Have you read about the Aerocivic? If it helps, he has probably done it.
Basjoos's side skirts are coroplast:
http://ecomodder.com/forum/showthrea...1-0-a-290.html
These members used rain gutters:
http://ecomodder.com/forum/showthrea...rts-10749.html