I have used a very simple Ortlieb map carrier on several motorbikes, including a vanilla 1200 and a 1200 GSA.
I use either one of the map cases or one of the chart cases, depending on what size map I wish to display.
http://www.ortlieb.co.uk/map-document.html
With just a very little bit of ingenuity, it's possible to use the cords to hold the very flexible case in place.
It does not scratch.
It is easy to read.
It is way less bulky than any tankbag.
It is easy to unclip when filling-up.
As it easy to unclip, it's easy to take into a cafe to study at liesure.
As it's double sided, you can spin it over to have a separate map displayed.
Cheaper than a tankbag, too.
Mine is a little different as it has a tag (to attach the cords) on each corner. A dig around on the internet will turn it up, no doubt.