I saw this Sling while looking for something else, and just had to get it.
It is just a simple strap with a carabiner like hook at the end. Plus a small loop attachment you put inside more or less any iPhone case out through the hole for the charging port.
Really great when you’re in a situation where you either don’t have a pocket and or having a bag isn’t practical. I use it a lot in the gym. The loop for it might not be the most durable, but probably not too hard to hack together a replacement.
I really like the concept of this product though.