Here is a mount that I created to securely hold my phone while biking. I created the mount to be the appropriate size to hold my phone when it is in its case, so unfortunately it is not that universal, but thought the design may help others make a similar mount.
Hardware used: #8 1" machine screws and nuts with washers.
Use 1.5-2.5 mm of outside layers and no infill to help it give it good flexible properties. I printed it in ABS.
To attach the two prices I used acetone welding and clamped them together to dry, any strong glue would most likely work though.