This is a phone gripper for a 1/4-20 mount screw.
Instead of using an additional screw-and-nut I used a rubber band. There's a groove for the rubber band to go on the top and bottom parts.
The top part has a mechanical constraint to keep it straight w.r.t the bottom, as well as a constraint to keep it from closing all the way down, this way the rubber band keeps its tension.
Sorry that I can't grab a pic with the phone in it, that's because I use the photo to take the picture. But you get the idea :)
Temp. extruder/bed: 210/60
Add a 1/4-20 nut and the rubber band...
Any kind of rubber band will do. Try not to crush your phone, while still providing a decent grip.
A 1/4-20 hex nut will be needed too.
Model is included for your remixing pleasure. It's not in very good shape, but you can make modifications still.