The best material to make it is a 300grams cardboard with one side black and otherside white. And have the inside white so that the flash would diffuse the white, while the blackside actually makes light only to come from the ring so you would see more of a ring shape instead of the shape of that ringflash.
Here are few pictures that were taken in Hong Kong at an Event, you can see the nice soft lights while also almost no shadow cast on the background. It looks way more professional than any event photography using normal flash with a diffuser. However using the onboard flash does use more battery and would not advice using it for event photography unless you have allot of batteries or a batterygrip. Click the pictures to see a bigger version.
Paper Ringflash by Chung Dha –