Here is my best attempt at an air dam. At least, the last picture I saved before something corrupted the file, which I never recreated.
The air dam is concaved to feed air sideways to the barge boards, which admittedly might should be bigger.
Based on observing OEM solutions, the outlet should be ±1 inch across the middle third of the wheel and the inlet can be any shape with a larger area to create a converging duct.