[Sven7]

You could get away with a more aggressive angle. Would this lower drag even more? We don't know. You should try enclosing the sides though. You could bring the sides all the way down to the bumper, creating a clean cutoff point for air. We don't know how much time the designers spent in the wind tunnel working on those corners, either, so testing is important.

Anyway. When making my lawn edging air dam I held the plastic close to the woodburning stove in the barn for a few minutes and it softened enough to bend better. You might try a hair dryer on its hottest setting.