Ive been thinking about it some more. I wish it was possible to find some different sized silicon straps to hold my phone. here's a set on Ebay: https://goo.gl/AVu7qX , but they are all the same size. They keep calling them universal and they are not. It looks like it would hold my bare iPhone fine, but with an otter case on, the strap I have is too small and is stretched too much. the other problem is the phone holder needs some kind of shock absorption. when I hit really rough pavement the vibration causes the phone to slip out of the mount.

PS. heres something interesting i've been thinking about:
https://www.ebay.com/p/rear-front-sh...p2047675.l2644

if link doesn't work its a miniature shockaborber set for a rc car. they are 4-5 inch long which is a bit bigger than i want. but they are pretty cheap and i've seen them in action, they are quite effective.