I fitted my Bestem 2012 onto my 400s using the mounting plate it came with, some spacers cut from scrap aluminum pipe, and a few nuts and bolts I purchased.
Just took a bit of thinking, a little time, and the drilling of four holes. All for under $100, including box purchase.
For my 650E, it could hardly have been easier. Spacers were provided with the 2012 for this, and the mounting holes for the existing backrest (removed) were used. I just had to drill one additional hole in the mounting plate, and supply the necessary length bolts.
I have also previously manufactured custom mounts from 1/4" aluminum plate to fit two "top-boxes" (Bestem 929's) as panniers to have a three box system on a C50., giving almost 150l of usable storage. (not pretty, giving a wide stance, but I wanted function over form for touring, and easily removable when not required).
Truth is, you can fit just about any box you want to, to just about any bike. It depends how much you really want a particular item, and how much effort you are willing to put into things.
I hope this helps.