I have a magnetic tank bag, don't remember the brand.
Has 2 "wings" with magnets in them that go on the top/sides of the tank, then has a strap to tie to the stearing head so it can't go backwards. I like it, as you can lean on it if you want/need to, doesn't get in the way, and doesn't move. Also, the "wings" fold up and clip out of the way, and there is a shoulder strap to carry it later.
they make the one i posted in a strap on version