Use a Crane motorized gimbal stabilizer with your smartphone using this Zhiyun-Tech Phone Clamp. Able to hold phones up to 6" in width, this clamp attaches instead of a traditional camera via a 1/4"-20 screw. If you prefer to shoot in portrait orientation, simply rotate the clamp. The rubberized grip keeps your phone in place while shooting.
Note: Not all cases will fit in the grip. The phone may have to be removed to be properly secured.