When I checked out the price of these diffusers I found they were a rather steep $22 (for what amounts to a plastic lid for your flash). While waiting around in a fabric store the other day (my partner is a dress designer) I started poking around in the craft section. I came across some noodle boxes (of all things). There was a selection of paper ones in various sizes, but it was the fancy translucent plastic ones that caught my eye. For $3 I could get a pack of ten and the small size looked to be a perfect fit for a small strobe. I grabbed a pack and when I got home, what do you know, they were a perfect fit.
I have included the build instructions in the comments although this is one DIY project that really does not take a lot of effort and can be completed in about two minutes! :)