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
Step 2: Dimensions of the Diffuser
This will fit a Nikon SB800 or SB600.
For other flash, maintain the 110mm width.
For the 250mm dimension, just ensure that the mat is long enough to overlap on the smaller side of the flash.
Step 3: Attaching the Diffuser
That's it :)
Step 4: Comparison
2. Direct flash, pointed upwards
3. Direct flash, pointed upwards, with built in bounce card
4. Light sphere
5. Light sphere, with dome
6 Ikea Diffuser
As it can be seen, results of the Ikea flash diffuser is similar to Light Sphere.
But would you carry a bulky vinyl bowl or a flat piece of mat and a rubber band :)