You're not referring to the effect of using a ring flash are you ?
Nope, but sure looks catchy, but very special and might divert attraction from the overall motive. I do a lot of macro with multiple flashes, and the direct fill flash rectangle is always too much visible in the eyes of animals.
Try joghurt becher with different colors
Thx, that's worth a try, I didn't get the idea myself because I'd suspected the plastic takes too much light away. A plastic bag sounds a bit too much unreliable because it's prone to change form or flatten(?).
Maybe you can add some black tape to a rectangular softbox to fake the look of a window cross.
Oh, well, that's also a good idea - seems I need to bump up my creativity with flash accessories a bit :-)
If your camera has a pc socket you can pick up a cheapo msnual ringflash on ebay.
My 60d hasn't got a socket, and neither has the 6d (thanks, Canon!) that I'll probably buy because it keeps being €1000 cheaper than the 5d3 and hasn't got the af assist bug ... and the pc terminal on the speedlite is incoming only (thanks again, Canon).