Id suggest looking at the Givi Tanklock range. You have to buy an attachment for your petrol cap and the bag just clicks into it. Makes it handy to filI up with petrol. I have 2 of them...the XS307 which is a 15 litre bag but to be honest it sticks out a bit far on the Mk2 Versys. The XS319 is a nice tidy one but it's only 3 litres.
They have a few in their range to have a look at👍