I know the OP got something already, but for others and those who may come later, Specialized has a rack mount for their nifty Stix lights which has EU standard DIN bolt hole spacing for horizontal mounting, and a single, center bolt hole for “reflector bracket” vertical mounting.
I use one (vertical mounted to a rack tab) and am very happy with it. It’s sleek, bright, super light, and easy to use with its integrated USB plug. The $50 Stix Elite is claimed to do 10hrs in 100 lumen Power Flash mode, and 22hrs in 40 lumen flash, and 77hrs in 12 lumen Eco Flash.