The phone holder consists of two parts hinged together, a block to attach the phone clamp to, the phone clamp (design by @Stamos, see https://www.printables.com/model/146560-snapping-handlebar-mobile-phone-clamp) and a flexible rod taken from an old broken car kit. The flexible (round) rod had a diameter of approx 12.6 mm. I've added the Fusion files for you to freely adapt. The hinged clamp parts can be printed as one (see the 3mf file), they will come apart easily and the advantage of printing them like this is that you don't need supports.
I added the design files for a phone holder, as an alternative to the adapter with Stamos' clamp. In addition I shortened the flexible rod (to not interfere with the shifter) and glued the parts (clamp & holder) together. Now it fits better for my phones, which both have a protective jacket. I've also updated the Fusion file to include the new holder.
The author marked this model as their own original creation.