oxford do a small tank bag for trail bikes but it is not cheap
baglux do a tank cover to which you can clip on a tank bag. even more expensive


From my own experience, baglux is the best solution if you can afford it


there is another brand (can't remember the name) where you have a pair of saddle hanging from the fuel tank.


I don't like magnetic ones because any steel in the proximity gets magnetized and you can also run the risk of scratching the paintwork