>>1653432at the very bottom of that pic in the middle is a boss to mount a light on. You just need an arm to push it out a bit, some bolts, washers and some nylock nuts.
https://velo-orange.com/products/vo-rack-to-light-bracketthere's an example. Nitto makes them too. I made pic related out of one arm from a centerpull brake filed down. Works good.