My husband's job sends us many, many cardboard boxes. I always save the double-thick ones for projects like this! There is one trick to cutting the thick cardboard accurately. I use a box cutter and draw a pencil line. The fist cut you make on the cardboard MUST be shallow and accurate. Then you cut progressively deeper until you cut through. I also use a piece of linoleum underneath. Cheaper than a cutting mat. You can even paint cardboard with acrylic paint without ruining it.