Cut from the plywood sheet two sections measuring 37 1/2-by-14 inches and two sections that are 18 inches long by 14 inches wide.
Cut two 2-by-4-inch beams to 6 feet in length and round off each end of both beams with a saber saw.
Assemble the baler box by connecting the four plywood sections with wood glue and wood screws.
Attach both rounded beams along the top edge of the 37 1/2-inch panels with wood screws every 6 inches.
Measure in 6 inches from each edge of the top side of both 18-inch box panels and mark. Cut notches approximately 1 inch down to thread the baling twine.
Measure 1 inch down from each notch and insert a wood screw.
Apply varnish with a paintbrush to protect the box from weather elements and warping.
Set the baler on a flat surface to load once the varnish is dry.
Cut two 10-foot lengths of twine. Wrap one end of each string around a screw below the notches on one side panel. Guide twine down the inside wall, across the bottom of the baler and up the opposite wall. Thread excess through the notches and wrap them around the other set of wood screws.
Pack the baler with hay. Straws should run parallel with the 18-inch panels. Fill to the brim, tamping often.
Unwind strings from wood screws and pull tightly across the bale. Hold with a finger the point where the strings cross and secure with a square knot.
Remove the finished hay bale from box by lifting the rails and setting the baler aside.