I saw a diffuser that mounts over the in-camera flash in a camera store. When I went back to buy it, it was gone *gasp!*. I had a wedding coming up to shoot, so I decided to try building one myself.
It consists of the following: 2 - 4"x4" cardboard squares 1 - 1"x4" cardboard support 4 sheets of 4x4 frosted drafting film (mylar) black photo tape small block of foam several rubber bands
I cut out a frame for the diffuser like a giant slide mount (remember slides? ;)) and taped together three sides. I slipped in the mylar and taped the last side shut. I taped on the leg, glued some rubber band bits to the inside of the leg and foam on the inside of the tab to stabilize it. My plan was to mount it on the camera with a rubber band, but ended up using more photo tape to hold it in place.