Here's another. I've always used a tank bag. I also always ride with the side cases and Givi top case mounted. I like to be able to stop and pick up most anything on the bike. (I also keep two sets of Rok Straps criss-crossed across the pillion seat, so I can pick up things like a 50-lb. bag of dog food or whatever. The tank bag is for small and nice-to-have-with-me things, some of them valuable, like my prescription sunglasses.)
On my 3 FJRs I've had one that attached with straps and hooks, one was ring-mounted, one Bagster bag that clipped in to the tank cover, and a couple of magnetic ones. I like the magnets best. Here's why. If I run into a store or somewhere and I won't be able to keep an eye on the bike, I like just sticking the tank bag in the big Givi. Also, it's the easiest--by far--to move out of the way to get gas. (I learned the hard way to put the removed bag up front over the ignition, not back on the tail somewhere, where some dummy might not see it and forget to put it back on and get 20 miles away in a ******** like Pahrump, NV before looking down and . . . well, never mind.) I also like that if I need to make a hard turn for parking or whatever, or even to lock the forks, if the bag's in the way, like big ones sometimes are, it's the easiest type to re-position.
Mine's a fairly inexpensive but fully functional Roadgear bag with a removable map pocket. Worth considering, IMO.