Never liked disc locks as it would be far too easy for me to forget that it's there. If I did though, I would either bungee to the pillion seat or put it inside a pouch on top of the bottom yoke.
I use a bumbag with integral visor compartment. Works a treat and have been using it since I first got the Ducati. No underseat storage, so this does a treat for map, water, fags, earplugs, and clear visor to keep the cops happy.
Hello u! I've got a Ducati tank bag lying about somewhere (red). I used to carry my padlock and chain in that. It's a bit tatty. But you're welcome to have it if you think it'll be any good to you. It's no good to me now I've got just four wheeled beasts, so it's looking for a nice home. I used to strap it to my bikes with the chain so that nobody would walk off with it and so I didn't have to carry it around. Helps protect the paintwork from the chain too. Let me know if you want it. I'll drop it down sometime.