Or you can always go with a gas cap ring mounted bag like the one Givi Tanklock bags or the ones from Bags Connection. They mount to a ring installed on the gas tank around the gas cap and they never touch the painted portion of the tank at all. They also pop on and off in a jiffy. The only downside in my opinion is the look, They don't hug the contours of the tank but instead they stand on top of the tank, which some might not like it but it looks fine to me.
I have the Givi T480 and I love it. Many on this forum have the Bags connection "City" bag and speak very highly of it as well.