A while ago @Hulsroy made himself this to hold his mudguard. I don't see why something similar couldn't be made to hold a light.
Interesting - is there any danger in having the metal round the carbon steerer?
You have a headset don't you? Pop it under the crownrace to hold it in place
You should definitely sand the edges smooth!
@gillies started
London Fixed Gear and Single-Speed is a community of fixed gear and single-speed cyclists.
A while ago @Hulsroy made himself this to hold his mudguard. I don't see why something similar couldn't be made to hold a light.