A couple of weeks ago I reviewed the Kenu Airframe, a nice portable smartphone car mount that cost $(removed) I mentioned that there was a knockoff version for $(removed). I ordered it and I like it a lot.
Like the Kenu, it has a rubber clip designed pinch your car's air vent. I drive a Prius and is fits snugly.
I have no idea why one of of the spring clamp is circular. Both ends of the clamp are rubberized so it grips the phone well.
It grips smaller phones as well as my large iPhone 6 Plus.
Another view of the rubber clip.
iMeshbean Air Vent Car Mount for Smartphone (10¢ + $(removed) shipping)