I found a Marsee Ovalution 10l tank bag with an expanding main section in perfect shape for $100 on the San Diego's craigslist, it's worked just fine.
It has a mounting base that stays put, while you can unzip and remove the bag. I love these bags, I have two of this smaller size since I bought my Honda RC51, one magnetic and one four point mounting system. For six years, I've owned a larger 20l magnetic one for the VFR. Build quality is second to none.