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.
Technical
I created with Lightwave 3D 2015 (the .lwo files), sliced in Cura, printed with black Hatchbox PLA, .1mm fine settings, 90% infill, .4mm walls, support touching base.
If you don't use Lightwave, be sure you download the STL files, as the LWOs won't be much use to you.
The phone mount prints upside down, and it was the right size first try with minimal cleanup (fixing some spreading at the "top" (which was the bottom when printed).
You'll need to provide a piece of strong 3M double-stick tape to fix the side of the mount to the side of the dash area, when mounting (shown in yellow in the perspective view).
You'll also need to find the rest of your vehicle mount parts, and drill the base to your own needs. The 90% infill should provide you with something solid enough to hold screw threads.
The Arkon shoe is an optional print - useful if you have a two-tab accessory to mount, but not so otherwise.
The shoe is a difficult print owing to having to clean out the support under the shoe tabs. You might do better printing it on its face (vs. on its back) and supporting all of the face _except_ the tabs. I don't know - haven't tried this.
The setup is more stable than my broken mount was, and perhaps stronger than the original piece.