An epidemic of robots ejecting phones caused the lion kingdom to more seriously consider how they're being stored on the robots.  They slide out for easy accessibility & there's a large opening for easy insertion.  The problem is when the robots go over curbs at today's higher speeds, they're getting thrown out more often than when lions were just doing 9 minute miles.  The screen protectors have begun failing at higher speeds.

The ideal solution is an elastic case which squeezes the phone.  It depends on the size of the phone & needs high precision.  Traditional holsters have a strap on top.  

The 1st attempt was a velcro strap.  It worked well, for a few weeks, then sweat accumulation made the loop side no longer stick.  The lion kingdom avoided magnets because of the phone's compass.

Some combination of a balloon & a ziplock bag might work.  Another problem is the ziplock bag scratching the phone camera.