Cardboard sheets of various shapes and/or with various shapes/sizes of holes. I often hold two cards at the same time. One with the shapes or holes, and a second card to help refine shapes, use as a "shutter" for the first card etc. Whatever you need to get the job done, just make it.
The carboards I use are white on one side and black on the other. White faces up to help see, black faces down to prevent reflection/flare.