There are hundreds of phone holders that will do the job.
I use the XGrip and Ram Mount Clamp below.
Like you I have a Garmin (XT) but I also like to run a phone as well when on tours as I do have a MRA Navigation subscription.
I find it useful as a back up and also for the waypoint colours.
For my Samsung S10 I have a full front and rear cover for when it rains.
I hope you find a suitable solution and have a fantastic trip in France.
Ram Mount and Xgrip.png