Migrated to:
https://www.printables.com/model/534466-phone-dash-mount-and-arkon-shoe-for-vw-gti-mkv-and
UPDATE: After a pretty good little road test, I finally ran into a period of leaving the car outside in the sun. PLA+Georgia Heat+closed car in the sun...really not good.
'These are not the plastics we're looking for'.
I re-printed with Inland PETG, 90% infill, and before doing so, tweaked the design to make the "flange" that tucks behind the dash upholstery 1mm thicker. I also deepened the countersink in the Arkon shoe, and created a "finger" for the Arkon phone holder; mine keep breaking.
Suggest you print two of the "fingers", pad them with foam if you don't use a phone case, so that you can keep their grip balanced - don't use one Arkon and one printed finger.
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My ProClip mounts kept breaking, and after re-hanging the phone on the dash about 100 times, I decided to design and print a replacement, perhaps stronger than the original part.
These files will print a pretty solid dash mount, and a "shoe" (not as solid, sorry. Improve it.) that allows you to slip on an Arkon phone mount (with dual tabs). Also pictured is a tilt/swivel adapter; it's not 3D printed, and not strictly necessary, if you can handle a fixed position, but should be available from ProClip (216009 or 215500 might be good picks).
You can drill the mount for your own phone-holding components.
I have no connection with Arkon, Brodit or ProClip, and do not own their trademarks or intellectual property. This mount and shoe are independent creations designed to improve on, upgrade, or work with their parts.