I bought a Pop Socket for my phone and once I put it on, combined with my Otterbox, the phone wouldn't fit in my Samsung phone mount, so I made an extension to attach to the arms on the side so that my phone wouldn't fall out while driving.

Print it laying down like in the picture for best results. It's a tight fit to go on the arms of the phone mount, but it should snap into place with some minor force and I can't imagine a scenario where it would fall off without a bit of help.

I added custom supports in Cura just between the hooks and at the corners of the curved part, should need more than that.