I use the Lumiquest, which tends to be a bit fiddly and probably can be improved upon.
When shooting close ups and when I have the capability, a paper napkin held over the flash works great... now that I think of it, I should just tape it to a hat and let it drape from there
I've also used one of those thin plastic/silicone cutting 'boards' - these are thin translucent things... white is good if you can find it, but I've used yellow and blue and pink... they make a nicely large diffusor. But you have to hold it by hand.
The yogurt cup is a good idea! In the US we can buy milk in quart sized white plastic jugs (and by the gallon too). I imagine fixing one of those would also be pretty awesome.