For those who've missed seeing it in action, here's the second, larger version of the tupperware flash. This one is a little larger than the original, so the light is even softer, although at 8" across it's obviously not as easy to pack along.
The inside of the pink part is sprayed with silver paint. Then I cut a rectangular hole in the bottom, folded back two flaps, and used electrical tape around them to hold a cheapie Hanimex flash in place. The diffuser cuts a couple of stops off the light output, but with faster film I haven't had a problem.