Just some pics of simple and really cheap diffuser for my 580EXII. The material is IKEA's desk liner - sold in 1,5m rolls for ~4$ (Poland).
I've put velcro on diffuser and flash - it's handy for other attachments too (color gels, anyone?) - for velcro config see photos.
Dimensions: 16cm x 50cm (width of the roll)
I've found one problem - it's too heavy for weddings, etc. - it often rotates speedlight end when switching position to/from portrait/landscape. So I'm looking for lighter material... bubble wrap maybe...
PS. Hooray for dirty mirrors ;)