Just had lots of grief with the Sat Nav in heavy rain, because it is currently mounted on a telferizer on the steering tube. The problem with this is that the touch-screen of the sat nav is not protected from the elements by the fairing, and has to face upwards to be visible - thereby exposed to particularly heavy rain that messes with the touch-screen. Looking at it - a ball mount could be bolted to the Mirror + Windshield bolts inside the cockpit, using the two bolts. But this would need to me made especially - no chance you could by something off the shelf. And it would have to be shaped - that is not just something made of straight lines. Surely this is ideal territory for a 3D Printed solution ? If anybody knows how to go about getting this done I would be interested.
Have you had a look at RAM mount? They have a million different mounting plates and brackets. I'd be surprised if you couldn't find something that would get it out of the weather.
I'm currently on holiday (!) in Austria and in the last few days have ridden through thunderstorms, floods, hail and relied upon my satnav on a Telferizer top yoke mount. It's a Garmin sat nav and the rain has no effect - perhaps it's because I fitted one of those screen saver films like you apply to your phone to stop scratches? A pack of several cost around £1 from eBay if I remember right (still on first one!).
Thanks for your post and good luck with your trip that I have been reading about on here. I went through similar weather in Northern Italy on the way to and from Mugello a couple of weeks ago, but my sat nav is the Tomtom 400. Never thought of the screen saver films, good idea and certainly give that a go. The other thing I do not like is not being able to read the small numbers very well when they have got big blobs of water on them - so there is that as well. Not sure what the numbers are like on the Garmin, but the Tomtom provides lots of detail that is quite small. So when the screen is covered with water droplets (like your helmet visor) it can make some of it difficult to read. At least the screen saver film would stop the rain from setting off random menus (when riding through traffic in Florence !) which would have been a great help !