I did find a few universal mounts with Type C cables built in but I am paranoid about those manufacturers using cables that are not spec compliant. I figure this allows me to install a high quality ( +Benson Leung tested) cable. 
I could not find a good car mount on Amazon that would allow me to keep my Nexus 6P in a case and allow a Type C to Type A cable to be attached (For Android Auto). I decided to print one.I did find a few universal mounts with Type C cables built in but I am paranoid about those manufacturers using cables that are not spec compliant. I figure this allows me to install a high quality ( +Benson Leung tested) cable.
Shared with: Public, Benson Leung, Patrick Campanale, Bill Bradford+1’d by: Andrew Bernstein, Alex Minnucci, Nikhil M, Patrick Campanale, Sir CharlesPatrick Campanale – 2016-02-05 01:25:50-0500Once printed and in use, I’d love to see how it looks / how well it functions.. If it works well, would you sell them?Keith I Myers – 2016-02-05 01:27:41-0500+Patrick Campanale I will let you know how it turns out. I don’t have a high quality printer so it will be a rough print. This will take about 4 hours to print.Alex Minnucci – 2016-02-05 04:29:24-0500http://www.thingiverse.com/thing:1170550Alex Minnucci – 2016-02-05 04:30:06-0500Shameless self promotion. It’s not even a great object! 😁Justin Jelinek – 2016-02-05 09:26:29-0500It’s a tight fit (and the review linked below was done w/ my N5, taking advantage of the Qi wireless charging) but the Tylt Vu car mount will hold my N6P in its case. It comes with 2 phone rests – the one in the pictures for the review, and another w/ two prongs to allow a cable to run to the phone.
http://techaeris.com/2015/04/09/tylt-vu-car-mount-review-quality-qi-enabled-car-dock/
Just an idea if your printed dock doesn’t work how you want it to work.Bill Bradford – 2016-02-05 12:06:52-0500I finally got one of those magnet-mounts where you put a thin piece of metal (adhesive) on the inside of your case.Keith I Myers – 2016-02-05 13:01:30-0500+Bill Bradford – For the 6P?Bill Bradford – 2016-02-05 13:11:45-05005X, but it works with any phone/case.
http://www.amazon.com/gp/product/B012EI4J3OKeith I Myers – 2016-02-05 13:12:42-0500+Bill Bradford – Is it sturdy enough that you trust it?Bill Bradford – 2016-02-05 16:20:24-0500So far, been using it about a week or so.