Nintendo Switch Pro Controller Holder for Phone and Switch
Attaches a phone or switch to the Pro controller. Requires a rubber band and 1/4-20x1.5 inch screw with nut (or M6 x40 would fit). If you don't have the hardware you can download models from McMaster-Carr.
Extend piece snaps onto main assembly and locks onto the square cutouts. Extend piece not needed if you are only using a phone. I would recommend to use the extend piece for larger devices and add rubber bands to top and bottom brace to prevent slipping.