I make them with fewer tools by using 1/8 inch baltic plywood, cut the two parts - the bigger square and the littler square, then gluing the two parts together with wood glue. I use a hand-powered jigsaw, a drill to start the blade on the lens hole, and a piece of sanding paper to smooth the sides. Takes about 20 minutes.
Shhh. Don't tell my wife I didn't need the router (and table, bits, jigs, etc)