I sure wish DiNotte offered a hard-mounted bracket like that for their lights. When I take the bike out for shopping/errands, I always worry about leaving the 140L on the bike held on by just a rubber band!
I've thought of cobbling together some sort of mount like you've got for the rear rack...ideally with the battery encased in some sort of plastic container underneath the rack. I wouldn't leave it on the bike in high crime areas, but it would be better than the current mounting hardware. As it is, anybody could walk off with the light and the battery with just a couple of seconds of effort. And it's a PITA to have to remove the light every time I run into a store for 5 minutes.
I've contacted DiNotte about this, but they don't seem very interested. Like most lighting engineers (going back to the NightSun* days), they seem arrogant when it comes to ideas from outside.
* I have an old NightSun Team Issue set that I recently tried to mount up for a friend. But, the brackets wouldn't fit the now-common "oversized" handlebars. I contacted NightSun and asked if they had a newer bracket that would fit this common bar size. The answer was No...and they had no plans to make one available either.