To adjust it for a given box size, add the desired depth and width of the box to arrive at the width required for the stock. If using pre-cut pieces for the top and bottom, use that dimension, plus the stock thickness, plus some reasonable dimension for play. In this case, the pre-cut piece is a 200mm × 200mm piece of two-color HDPE, so 8.125" × 8.125" for box dimensions, and the stock area should be 16.25" wide: