Ok, the phone mount is ready. I made a quick test on another tree at ground level:

As you can see the phone is held by a bungee strap, which holds the device firmly against the wood. When you look closer, you can see that at the edges there is a groove of about 2cm between the two bumbers, so the phone is also not able to slide down. The app I am using has the name "Accelerometer Analyzer" and is able to log the values to a CSV file on the SD card :) Now I have to wait only until the phone is fully charged and the data collection can begin!