I'm using this iOttie Mount Holder with my iPhone 6 and I'm really happy with it. very safe and easy to use and basically answers most of my needs.
Amazon.com: iOttie Easy One Touch 2 Car Mount Holder for iPhone 6 (4.7)/Plus
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Highly recommend the pro clip. I got the one with the lightning pass through. Adjustable for your case size. Easy to install and very, very strong. Doesn't wobble, etc. Good product.
Are you using the mount with the two adjustable sides or the mount with the non-adjustable sides but with the adjustable clip-top (~Seidio clip)?
What are the pros/cons of the two designs?
[Edit: I should add that I plan to use it with the phone in a horizontal position.]