I've seen jogger arm band cell phone holsters that could probably be adapted for your pack.

I have a couple of different ones that I use. One leather/vinyl one that attaches permanently to the bars, and the phone slides into. It should snap, but the phone is a tight fit, so I just leave it open on top. Sorry, I don't know the brand.

This silicone one seems to be reasonably secure. Hasn't tossed out my phone yet. Probably best to mount on the stem.

https://www.bulbhead.com/nite-ize-ha...bar-mount.html