How the !§$%&!!! do you carry a M18 or similar motocycle D-lock?

I'm not very mechanical, icke, but I wouldn't have thought that there is a possible solution than a custom-made mount. I'd go for that if you're determined to stick with the same lock. Some people bungee their locks to their carrier rack, but I'd guess you might not have one. I think that a motorbike lock is overkill.

It sort of seems like overkill, especially when you're carting the fucker around (along with a mini-D for the front wheel), but I often need a long shackle due to my hack fixed being a mtb - clearances in the rear - and it was on offer when I was shopping for something. I only use it for locking up around town. Not in my 'trusted places'.