This is a MagSafe bike mount for the stem.

I added a pause to the print in the slicer after 1.2 mm (right click on the “+” on the scroll bar at the right side of the window) to add the magnets. I used these: https://www.otto.de/aftersales-orders-ui/productdetailpage/028669a1-6bc2-4b42-8c4d-705d7b6757ae

CAUTION: USE AT OWN RISK!

I use a silicon case with MagSafe. It is so strong I can even ride down stairs while the phone is attached to the mount. Do not use it without a silicon MagSafe case. The silicon increases friction a lot.