I will try to post a picture of my tail light mounting.

I use a cross bike for commuting so I had to use a slightly different moutning technique due to canti-lever brakes.

1. I cut a piece of PVC dow to the correct size.
2. Drilled a hole in the pipe so that I could bolt the pvc pipe to the brake bridge. (I used locktite to secure the bolt in the brake bridge threads)
3. Use two zip ties to support the PVC pipe against the brake bridge