I use an Oxford sportbag, never had any problems, magnets hold very well and I had more weight in it than the panniers on holiday since we put all the guide books and camera's in the tank bag. Word of warning-it is possible to cause fuel starvation if your bag is VERY heavily loaded since the breather for the tank is round the edge of the fuel filler and the tank bag covers it. I will leave you to work out for yourselves how I know this..........
If you want a bag a quick trip to a Riossi store could be profitable, they are selling everything off at 50% off.
John