have a rummage in your LBS for a piece like this, bound to be something like this from a rack? The Nitto RH clamp could be straightened out too, and clamped between the stay and the rack as shown above.
Failing that trawl SJS, they almost always have the widgets I need for things like this.
Looking for methods to mount my Luxos U on either a nitto m18 or tubus tara. Found this but it involves buying three different mounts:
http://biketouringnews.com/touring-bike-accessories/lights/buschmuller-luxos-u-head-light-mounting/.
Also i've seen people repurposing caliper brakes but it seems to require a bit of metal work.