How should I color-pattern a circular shield to create an optical illusion?

I am embling a flat circular shield for a foam-combat game, and the idea of covering the fabric face of it with an optical illusion occurred to me. What sort of simple pattern would produce that sort of effect? A two-tone pattern without gradient-shading would be easiest (white on black, single color on solid background, etc). The optical illusion should ideally visually distort the apparent size or location of the shield, or be difficult to look at without the eyes playing disorienting tricks in some way.