While it isn't such a big DIY thing, I thought it might be worth mentioning. Here in Germany, Aldi is the king of "discount" grocery stores. They have a line called "Be Light" and one of the the products is a yogurt drink. It comes in 1 liter size plastic bottles.
The best about these specific bottles is that after you peel off the plastic outer label, you have a pure white, thin plastic bottle. I took a pair of scissors and cut off the neck. I tried it for size and while it is a bit small for my 430 EZ, it fits pretty well without anything on my 420EX.
I am pretty sure one could figure out how to split the sides and slide it onto the larger Speedlites or build an adapter of some sort.
I tried it out on some close up "product"-type shots and was pleased with the results. Nice, soft light - thick enough to diffuse nicely and thin enough that it doesn't waste the light - it is also light weight enough to stay without fastening. Now to keep experimenting and adapt them to the other sizes.
Probably would work with any similar size white plastic drink bottles.