I was inspired by your post to head out to the garage and rummage through my collection of left over mounting hardware. At first I tried reusing the mount for an old Niterider 6v system but I kept running into problems. But then I hit on this combination:
I have a large tube mount that fits through a helmet opening. It's very secure and doesn't interfere with the helmet fit at all. The second piece mates with it and can be adjusted to get the vertical just right. I have my Romisen in it because my P7 is on the handlebars, but this would have worked with the P7 as well.
Here you can see the components. The light is held in by a no-tools screw making it real easy to remove. The piece that is left over registers on the fred-o-meter but I don't think it pegs it. It's a good compromise solution.
