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Denis55: After considering many diffuser DIY options, including Brian's excellent Coke can version, I decided to build mine from an original design. (mainly because I didn't have any soda cans!) My only concern was to have a relatively large lit area, and a fairly long design so that the light could bounce around a bit. If the flash doesn't reflect off the sides, it will just create a bright spot in the center of the paper towel.
I meant to build a prototype and then document the process on a second version, but the first is working so well I may never get to it. I used a cardboard box, aluminum foil, clear packing tape, a paper towel and a rubber band. I'm attaching a VERY CRUDE drawing of the dimensions I used for a 430EX. (I've got the 580EX as well, but find the smaller flash more than adequate and easier to use in the field).
I made two of each size on my diagram, then lined the inside of each one with aluminum foil. Be sure to have the shiny side out, and lay the cardboard piece on the back of the foil. Trim it a half-inch larger all the way around, wrap and tape it. Finally, lay the four pieces face down, alternating sizes, and align the angled edges. Run a strip of tape connecting the edges, then complete the "box." Put the paper towel on the front and secure it with a rubber band. Double up a rubber band around your "flaps," slide it on the flash and you are good to go!
Please disregard the terrible drawing!