I've provided four sizes, 60mm, 80mm, 100mm and 120mm. The pictures show all but the 100mm. I printed in ABS with 3 walls and 10% infill and they seem the perfect balance between strength and flex.
Thanks to the others for inspiration. Please improve on my design and post a derivative!
UPDATE: 18th Sep 2012, I've added new versions (v2) that have an improved larger clip. I recommend using those instead of the originals.