Jason,
According to Rieger guide, the front and rear aprons are held by nothing but few rivettes and silicon. I fabricated simple brackets out of aluminum piece. I used two brackets on the front, and 3 for the rear.
The side skirts can be installed using double tape and silicon. Rivettes would help too.
Overall, its pretty straight forward installation considering the works you do for your c220.
How's the car running??