I did essentially the same as this but using a Givi bag. Works great, the only issue I had was that the clear screen cover was randomly pushing my sat nav screen making it open up all sorts of settings whilst riding along which blocked the map, which is quite annoying as you ride up to a roundabout/motorway junction!

I simply put a piece of perspex in between the sat nav screen and the clear screen cover = problem solved. Although it creates a bit of extra faff if I need to change anything on the sat nav.

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