Although made for the new line of BMW scooters, this bag looks like it might work on the Burgman 650 at least. Yes, you will have to contend with the BMW logo, so hopefully that means your Burgman won't break down more. 
More info http://www.webbikeworld.com/bmw-c-650-gt/tunnel-bag/.

More info http://www.webbikeworld.com/bmw-c-650-gt/tunnel-bag/.