In the various practice rides I’ve had the phone in my front pocket (when I have a coat on) or a back pocket (when I don’t) to do the navigation. This works up to a point, but it means whenever I want to check the route or take photos I have to stop and rootle in pockets.
So I’ve just bought two of these:
https://www.quadlockcase.eu
They are apparently the best, easiest, toughest mounts for phones on bikes, and super-simple to mount on the bike too. I think they are going to be great, watch this space for comments…
PS (many months later) They are great. I recommend them unreservedly.
DISCLAIMER: No, they didn’t pay me anything to say that.