Actually this rear bag by KTM is made for enduros and supermotos but with a little modification I can mount it to my Duke 690 as well. I chose this bag because it changes its size relative to the stuff I put in it and it is waterproof.
Mounting to the Duke 690
I fixate the bag using the pillion's footpegs. Unfortunately the delivered straps are not long enough to reach the footpegs of the Duke 690. So I built two with the correct length on my own. For that I got a roll of black 20mm fabric belt, took 2 of the plastic clips of the original straps and sewed loops on their ends.
The delivered loop strap is attached to the bag and pulled over the the pillion seat's lock mechanism and its supporting points. After that you can put the pillion seat back in place. The rear bag is now held by the strap and the seat itself.
The self-made straps are put around each footpeg using the loop at the end. After tightening you clip them to the rear bag.
I'm very pleased with the KTM rear bag. At all times the bag stayed on the rear of the bike without moving at all. Even at high load there was no wobbeling. Because of its design it changes its size according to the stuff you put in it and it is water proof.