Cut 2x3's into 4 foot lengths. Cut 1/2" plywood into 15" widths, 4 feet high. Cut out rectangle from center of each, measure twice the width of 1x5 from bottom and top, and 4" in from the sides. Cut 1/4" plywood to just larger than these holes. Using construction adhesive, glue 1/4" plywood to inside of each 1/2" piece.