I thought I had a good plan: Order a few ABS plastic project cases from All Electronics, cut out holes on one side of the boxes for the flash heads of my MT-24EX and the other side where I'd mount a piece of diffusion plastic and presto! I'd have a brand new diffuser set. I could easily change the diffusion material just by taking the front of the case off and putting a different plastic on it (great for experimenting).
So over the course of three evenings I became one with the Universe and a Dremel Tool. There were so many plastic shavings scattered all over the kitchen it looked like I had tossed a manikin in a wood chipper. I'm a man who knows his limits so when I wanted to test two different box sizes I ordered three of each, even though the MT-24EX only has two flash heads. After completely ruining one box, and the bottom of another, I had two sets of diffusers built and I tested them against the set of Puffer diffusers that I built over a year ago. During testing I ran into one minor snag...
The new diffusers didn't work.
Well, they did "work", but I couldn't get light that was more diffused than the Gary Fong Puffers that I've been using (if you use that link to order two Puffers you'll only pay $30 USD + shipping). Looking back on it I think I'd have to build a diffuser that has at least twice the surface area of the Puffer plastic. Unfortunately the resulting diffusers would be too large to use with the flash heads connected to the flash mount that Canon supplies with the MT-24EX. I'd have to get the new diffusers further away from the lens, and it would just make hand holding the rig more difficult. So for my style of shooting the Puffer makes a better diffuser.
Here's a video showing what the Puffers look like and how I use them: