Yeah, those "rubber bands" are terrible. I never used mine. You can buy extra mounts on Amazon for $13:
https://www.amazon.com/gp/product/B00ET29I1C/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1
I have three so I can move my Garmin from bike to bike and they work well. I'm not going to bother with the leash. These mounts work well enough to use on my XC mountain bike without worrying about anything falling off.
It is a piece of plastic. Don't pay 40 bucks for it. This works great