I bought a Chinese bike pump and it came with a phone holder to frame-attach it. I have been using the holder for my phone. It's entirely cast of one piece of silicone rubber. Four stretchy corner straps hold the phone, then there's a cradle under for the bar, and a integral strap that feeds around under and through and hooks like a lot of silicone light mounts. I found them on Ebay for like $8. I had resisted a phone mount because all of the "good" ones look like they hold the phone too stiffly connected to the bike and I was afraid of my phone getting shook to death. Also, anything that has screws, etc can be shaken loose. This one, however, is floppy and soft so I feel like the phone is more shock-mounted and safer. It's been working well. My Samsung A50 is a tight fit, but it works.
It's similar to this:
https://www.amazon.com/Bovon-Univers...dp/B07NV55BZT/