I'm sure a tight belt is a good solution for most people, but I'm going to run right into the same problem I have with armbands - tight elastic is going to trigger my eczema as soon as I start sweating.

And then even if a phone weighed absolutely nothing, I've still got to look at the screen to control it, which means either stopping motion or taking my attention off where I'm going, it's still got to be set to the highest brightness to see anything on the display, and as soon as I'm sweating, the touchscreen becomes almost useless anyway.

Do what works for you, but at the end of the day, I will never be satisfied trying to square peg a general purpose smartphone into the round hole of a run. Devices that were made specifically for this purpose and did it brilliantly used to exist, and don't now.