Mounting case for a Pixel 2 designed to accommodate the phone with the Newdery auxillary battery case found here :
The mounting bracket was designed to repurpose an old handlebar mounting bracket for a previous phone. It is glued into the main body and can be modified for other applications.
The case is held together using silicone rubber bands for hairstyles (ponytails, etc.) and can be found commonly in drugstore cosmetic depts.
This case could work with a phone that doesn't have an aux battery, but you'd have to put some foam behind the phone to fill up the unused space.
The Fusion 360 file is kind of a mess, as I had several iterations of the design all contained in the single project and I ignored good design practices for making things easier to modify. Sorry 'bout that.
Update: Version 2 has a built-in sun-shield and also prints in just two parts - no separate buttons for the rubber bands.