I keep mine in a top tube saddle bad....only I put it behind the seat post on the arms of the rear rack.

Something like this...
https://www.amazon.com/dp/B01EWZ3HC6...ing=UTF8&psc=1

There are a million shapes and sizes and colors. On my old bike I kept it right behind the stem on the top tube but it just didn't fit well on my current bike that way so I found the alternate spot behind the seat.

Most have a case for the phone that sits on top but I never liked those.

I have a foam liner I custom cut to fit into one side of the bag so my phone is well protected while I ride. The other side is open and I keep my wallet and keys and stuff in there.

I leave the bag on the bike at all times. So there is always a phone storage option and a place for my keys. Never have to think about if I have pockets or if anything will fall out. Nor do I have to rely on a full pannier or trunk bag, which I never use when just riding for fun on the weekend.

And I keep a shower cap tucked under my saddle so if it rains I quickly put it out and coverage both sides of the mini saddlebag and then my phone is dry and so is my car's keyfob. (3rd image)

(The phone on the handlebars in the second picture is a dedicated phone for mileage and mapping. I hate the idea of my daily use phone being on the handlebars.)