Instructions here will fit on a Nikon SB800 or SB600.
Alter dimensions accordingly to fit other flashes.
***Update 8th Jan 2008***
Just came across a commercial product that is similar:
Step 1: Materials
1. Rationell Drawer Mat (from Ikea) - the diffuser
2. Cutting Mat - for cutting the drawer mat
3. Metal Ruler - for cutting the drawer mat
4. Pen Knife - for cutting the drawer mat
5. Marker - for marking out
6. Rubber Band - for securing the diffuser on the flash
7. Flash for fitting