Drill and be done.
I presume you have seen my install about the side ducts. Drilling gives you the best means for attachment. It may seem scary to drill, but who cares. These scoops will be on the car when you eventually decide to sell the vehicle. Do you really expect to ever remove the side scoops? Make an install that will be structurally sound for the next owner as well. I do not worry about my installation moving at any speed, considering the pressure of the location. It's an air intake under a lot of force and the lower factory bracket is quite nice.
Elise: My process of adding Exige side scoops