Just get one of these and then use your usual handlebar mount on it.

https://www.amazon.com/Origin8-Eyelet-Stub-Black/dp/B00629QAVG/ref=sr_1_1?ie=UTF8&qid=1522884588&sr=8-1&keywords=origin8+eyelet

I have these installed in the eyelets on both my front and rear racks so I can mount my head/taillights to them.