I designed this bracket to hold my iPhone while it's in the case, and allow it to be mounted to various tripod brackets.
The opening of this clip is 68mm, and the loops for a rubber band allow slight pressure to keep the phone in place.
There is a spot to glue in a 1/4-20 nut which will allow it to mount on a standard tripod.
You might want to add foam pads on the edge to protect your phone, but I use a silicone case, so it's not required.
There are other iPhone clips around, some much more elaborate and adjustable, but I just needed something simple and specific for my setup.
You can see the video documenting the design/printing process over on my YouTube channel: https://youtu.be/9lnWRDb6DEI