I use the sat nav on my mobile and had it mounted on the top bar. It was so light that it shook like a lamb's tail. Tacking a sheet of roofing lead (no, not from a church roof!!) on the back improved things greatly. Now i fixed it in the centre of the bar and like it much more. Even without the lead. Stays dry from above in rain as well as long as you keep above 40mph. But it gets wet from behind due to the draft coming up through the fairing. Will sort of box it in for the next tour.

Rob