I created a sturdy mount for iPhone SE to mount a smartphone to a MTB while shredding the trails. The phone can be used for directions from a downloaded GPX file. All buttons are accessible and the speaker can still be heard.

The mount is made of 3 pieces: 

• bottom piece
• top piece
• phone mount

The top piece and phone mount are glued together with epoxy for a strong bond, and so everything can be printed without supports. The bottom and top piece are fastened around the steering tube with 4 M3 bolts and screws. I added some old inner tube for more friction and paint protection. Printed in PETG. 

The steering tube of my bike has a diameter of 31.7mm, but I linked to the original model in OnShape you can customize to your own dimensions, or design a mount for your own phone. 

https://cad.onshape.com/documents/44e5e66f6b53aeb7e4a3a96d/w/2551baf1b726488eb25eab1d/e/458fce962f5bc82aa139f9c7?renderMode=0&uiState=648cb197c3e7be0aeaf1d7da

Tip: When printed in PETG, if your phone is not the perfect fit at first, you can heat up the mount with a heat gun, carefully slip in your phone and nudge the mount to a proper fit. Once the mount cools again, you'll have a perfect fit. In my experience, the phone will never slip out, even on rough terrain. Still, you'll do this on your own responsability ;)