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Cut wood to size. Window boxes are generally slightly wider than the window frame. One materials option: pressure-treated plywood. Cut 45-degree angles on all ends. Use wood glue in corners. Follow with 2" deck screws. To trim window box, glue and nail 1x1 molding to the front of window box and then cut two cross patterns for the box. To attach cross fretwork, cut two battens to size. Hold battens diagonally on the box. One goes from upper right corner to center of lower fretwork, and the other goes from upper left corner to center of lower fretwork. Mark the angles on the battens so they will fit between the horizontal and vertical frame. Cut, glue and nail battens into place. Cut two more battens to create two more diagonals. Cut them in half and cut them to fit. Glue and nail battens in place.