Got bored and decided to come up with a solution for holding the sat nav that does not rely on maintaining a cooperative passenger.
I made up two bungee mounting bobbins on my lathe that use the existing gearbox trim fixings. Gives a reasonably placed sat nav that allows for hands free cornering and fairly well protected from the weather.
https://www.dropbox.com/sc/h0gr1r73xwq46yq/ktdACe3RwT