Below is a photo of how I mounted my Abus GRANIT XPlus 540 u-lock to my rear basket - it's the only place on the entire MiniST's unique frame that works (the ABUS cable is for occasional use around trees and the like). I know the 6500 is a folding lock, but may have a mount included for attaching to a frame bar like mine.
I think the 6500s are typically mounted to water-bottle stud locations like the ST's front downtube; you can also buy adapters to mount water bottles to the front-rack interface; I have one of these to hold two bottles and you could mount the 6500 to one side:
https://amzn.to/2ZkSjwIWhen I was researching locks, I found more than one video showing how to blow the 6500 apart at a folding joint (just like all the other folders), so I went with the u-lock design instead, and the 540 costs less as well. It's certainly bulkier and heavier than the 6500, but in this location is completely out of the way yet easily accessible when needed.
If you're not in a high-crime area, the folders are probably a great option. I live in a peaceful little town in the woods, but my recovering-engineer OCD wouldn't let me get the 6500 I originally wanted after I saw it defeated with a little horsepower.