I finally bought a tankbag for the Duc. The model is “Engage-XL” and I got it from twistedthrottle.com along with the tank ring. This is a quick-detach tankbag that comes with a rain cover and a shoulder strap.
The tank ring mounts to the ring surrounding the flip-up gas cap. It looks like a horseshoe with the opening towards the back of the bike. The bag itself has a mating horseshoe that fits over the tank ring and clips on with a spring-loaded latch. The underside of the bag slopes down towards the back and is a reasonably good match for the Duc’s tank shape. It takes all of 5 seconds to attach or detach the bag.
After seeing how well this fits, I am kicking myself for not buying it sooner. Now, I can take a spare pair of gloves, a disk lock, my phone, a rain jacket, and a camera with me without having to stuff all of that in my jacket pockets. There’s still a fair amount of room, even with these items in the bag – the specs say 14 liters of space.
This will definitely make dayrides on the Duc much more enjoyable.