Here's a couple of photos of the shutter I'm currently using. It's made from a couple of bits of scrap card. One has a quite large hole cut in the centre which fits over the pinhole (I didn't cut this very neatly - you can do better!) The other piece acts as the shutter and slides behind this, covering the pinhole. I stuck some black tape on the back of this shutter card.
Shutter closed.....
shutter open.....
