Been meaning to share this... when you're in a pinch and need something you don't have. This works quite well!
That's a great idea and one I've been using since the seventies. I made mine out of white plastic cut from bleach bottles or similar items and held them on my flash using a rubber band. They work great for kicking a little extra light forward when using the flash in bounce mode. I still have a few of these things around the office somewhere but, since Nikon started putting their own bounce cards on flashes, I haven't had much need for them.
Those plastic cups are also handy for stopping flash light from spilling out of light boxes or, in my case, octaboxes when using speedlights. I get the black ones you can find at party stores.
--Ron