Search for [universal mounts]. Some grip the phone with a silicone harness (
ex), others use a glued universal adapter (
ex) to fit a quarter-turn handlebar mount. (I do not use such system so can't vouch for their reliability. I own a Quadlock bundle which works well, so I assume that their universal adapter is a reasonable solution, although it'll add a rather large hump on the back. Depending on your circumstances, you may want to purchase a small bumper for everyday use and switch to you case during outdoor activities)