Adhesive tape. The bottom was moulded from the fender surface to lay nicely on it. The fender edge is about 3cm wide. I used adhesive tape and tried to remove it and damaged the paintwork on my fender. I doubt the wind will tear it off.
Given the angle of your lens, the scoops seem VERY outward maybe... Just to be sure, are they "in line" with the car's "rectangle", like say the elise/cayman c/f scoops do, or do they stick outward like a trumpet would?
A picture from above and head on from a distance (given enough resolution, you can crop and resize if too small, etc) will help correct the lens distortion, if they are "in line".