The Cortech magnetic bags look and work very well, available in several sizes. I usually use the 6.5 liter (I think) Super Mini bag which holds my camera, watch, wallet, cell phone, and a gaggle of other stuff. They're well made, affordable, and easy to find, lots of good used examples are normally listed on Ebay
I also on occasion use a flatter, expandable RKA bag, also magnetic ( the only way to go, in my opinion, with steel tanked bikes), but it doesn't "seat" quite as well with the tank cover in place. The magnetic base doesn't clear the outer cover pockets very well, but it sticks like glue when the cover if off.
Which ever way you go, unless you just have to buy new, you'll save about 1/2 by getting a gently used bag, that'll last for years.