Thanks to the helpful people here, I combined ideas from 2 threads for creating a flash diffuser.
http://forums.dpreview.com/forums/read.asp?forum=1025&message=10645209
http://forums.dpreview.com/forums/read.asp?forum=1025&message=18308364

I bought some Fun Foam or Craft Foam from the dollar store and cut it out based on the template in the first thread above. I applied velcro strips to the flash and foam. That's it. Simple, cheap (~ $2.00), portable. 20 minutes to do.

The template design adds stability to the diffuser because of the side flaps. For this reason, you do not need to add an extra layer of foam or sandwich cardboard, or anything else between it, as suggested in the other threads. One sheet of foam works great.

Here are pics of the finished product.

http://www3.sympatico.ca/brendan.hennessy/pics/diff1.jpg
http://www3.sympatico.ca/brendan.hennessy/pics/diff2.jpg
http://www3.sympatico.ca/brendan.hennessy/pics/diff3.jpg
http://www3.sympatico.ca/brendan.hennessy/pics/diff4.jpg

And a sample pic taken with it... Pretty good light from a manual, 20 year old flash - Vivitar 285HV. (Taken with D50, manual mode, 1/60s, f4.5, flash set to f4)

http://www3.sympatico.ca/brendan.hennessy/pics/285HV.jpg