Thanks for sharing. This is brilliant; very cool and elegant! As a bit of a gear head, it makes me flush a bit. What kind of paper did you use? Do you find not using a hood at all problematic? How far would subject typically be when using this rig? I have the older f/2.8 macro which has a longish filter. I guess I could extend the upper panel and place the collar around the end of the hood.
Tom, there are many materials that you can use for the fan-shaped diffuser. I started out using writing paper, then moved to vellum, and now use a translucent plastic stationery sheet sold by Staples. The latter is great because a sprinkle of rain doesn't bother it.
If you are thousands of miles away from home and lose your diffuser, you can make a new one out of writing paper.
"Not using a hood?" I don't use a lens hood when shooting macro with the 100 L. I always use a hood in other situations, but not with this. The hood would seriously encroach on limited working distance. Anyway, it is not needed to control stray light here.
Since I made this, it has evolved. Instead of a reflecting panel on the flash head, I now I use a snoot. It is just a larger panel but with a bit of a wrap-around shape. Its front touches the edge of the diffuser and is attached to it using bits of Velcro. This stabilizes the setup and gives it a fair bit of resistance to a mild breeze. The snoot is made of flat plastic sheet with aluminum foil glued to it with contact cement.
A picture tells a thousand words and I will try to upload a recent pic of the rig soon.
BTW, I always use the 100L with a Kenko 20mm extension tube. That gives me around 1.2x to 1.3x magnification, which is nicely useable for hand-held shooting like this. AF still works OK if a bit balky at times. With more extension, the AF is not very good.
Working distance is around 15 cm (6").