Get yourself a 4" chunk of pipe that is about 7/8" OD (PVC, copper, etc). My local hardware store sells PVC pipe in that diameter for about 10 cents/foot. Buy a couple of feet and you'll have enough for several bikes.

You can mount the pipe to an angle bracket or one of those Nite Rider Universal Mounts. You can then attach that to wherever you'd like on your bike. I've attached my NR mount to a seat stay, a seat post (each for the DiNotte 140R), and a handlebar (for a MS and/or DiNotte 200L mount). I think it would also clamp onto the fork just fine, if I wanted to. Rack tubing is probably too small in diameter, so for if that was your light's location, I'd affix the PVC pipe directly to the rack.

The angle bracket could be mounted directly to a braze-on for a horizontal mount. It could be mounted to the front reflector-mount hole on a rack for a vertical mount.

How do you mount the pipe to the NR mount, angle bracket, or rack? If it was going to be screwed on, I'd drill a hole through one side for the screw to come out, and drill a larger hole in the other side for me to fit an allen key/phillips head through and then screw it onto the mount or rack. I've used zip ties successfully as well. On one bike, I have a 6" chunk of PVC mounted to the front rack. An end mounts to the middle reflector-mount hole, the middle is zip tied to the rack, and the end hangs off the front/right of the rack and the light is mounted there.

All of my lights are now O-ring mounted (140R, 200L, MS900, Baja Designs Strykr). I have ridden thousands of miles with them mounted to a short chunk of PVC tubing with no ill effects. For a while, I even had a chunk of PVC attached to my helmet via tape and mounted my MS900 to that. It provided a cheap, effective, low-profile helmet mount for me.

Tip: wrap the PVC in a layer or two of vulcanized electrical tape to make the light less likely to slide around. I get it at my local hardware store for less than $4/roll.