I use a Proclips base, with a nite ize steelie phone mount on it. The steelie ball mount is what I had in my last car, just glue'd to the dash (comes with 3m adhesive pads) - but found no good place for it in the i3, until I came across the Proclips base mount. The iPhone attaches to the steelie with a small strong magnetic socket that is glued on the back of my Otterbox phone case, with a 3m adhesive patch. Both steelie and socket are easily removable - leaving no marks, and can be re-glued with replacement 3m patches. I have moved the socket to three different phones, and the steelie has moved two cars.
2015 i3 Rex, Capparis White, Tera World, Technology & Driving Assistant, Parking Assistant, Harman Kardon Audio System, 19 inch 427 wheels, EVoInnovate EVSE