Being an avid mountain biker, I have come to love the RokForm Cases. They are durable and offer many ways to mount the phone (Magnet in the case, as well as the original quad-locking system.) While biking I find that using an Iphone for all video and photo opportunities serves advantageous, being that you don't need to transfer files. I would often times when I did use a gopro like device place it in my mouth while filming people (the perspective is right on). Now they have mouth pieces you can buy.
Though some often carry too much movement making the video bounce.
An Alternative is this: https://www.thingiverse.com/thing:933751
What started off as a simple need to create a mouth-piece for my iphone, has evolved for this multi-use mount that can be used on the gopro mounting system. I use this even for my Mavic Air Drone while using the mount created by Thor https://www.thingiverse.com/thing:2885445