I made my own smartphone mount using the reflector brackets that came with my bike.

Taking the reflector off, I epoxied/bolted a 2"x3" piece of 1/16" plastic to it, then covered the plastic with hook velcro. I then put matching loop velcro on the back of my phone case.

The modified mount is on my handlebars, and tilts up/down to position the screen just right. The phone sticks to the mount very securely, but I do use a #87 O-ring around the phone/mount as extra security.