I installed mine using the factory mirror pivot bolt. The aluminum spacer is 16mm long, cut from a 3/4" tube, available at the local hardware store. I had to file the PVC cap down a bit as well, and give it a notch to clear the Vitaloni base.
The tension that the washer/PVC cap creates will hopefully stay put, as it allows for easier installation of the base to the door. It allowed me to pivot the mirror out of the way to tighten the base bolt.
Have to admit, the view is a bit compromised, at least in the garage. Looks neat, though, and is a legit tie to the 934 and 935... Hopefully it's manageable on the road. Will find out when the rest of the car is done I guess.