I made my air dam reach down equal to the lowest ground clearance situation already existing under the car. So my ground clearance didn't change.
I cut a horizontal plywood panel to fit directly behind lower edge of air dam. Extend it back as far as practical. This creates a smooth zone of airflow where that panel is. Attach horizontal panel to dam with small angle brackets + screws.
I'm TOLD that an air dam really is best when you also have side skirts, to help maintain the vacuum created under the car. I haven't gotten around to making them yet. Wife thinks I need to tend to career and house. She's always right, as it turns out.
Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.
if you're still looking for Coroplast, look for temporrary signs, over here it's often used to put some info of the contractor on the fence of a construction site. sooner or later these signs might end up with the trash so you can have some fairly large bits for free...
do you have any pictures of what you made? i'm still looking for inspiration.
Home depot sells aluminum coils. Its for making vinyl siding trim. They come in a cardboard box 1'x1'x24". I think its 28 gauge which is 0.0113" thick. Its what I used. The heavier material you use the less framing is necessary.
02 Saturn SL
for pics click the link below