Using 3/4-inch plywood, cut two pieces 6 feet by 4 inches for the side rails and two pieces 30 inches by 4 inches for the top and bottom rails. Cut four plywood supports for the seat 1 1/2 inches wide and 1 1/2 inches shorter than each of the rails. Cut the seat from plywood to fit the interior measurement of the rails.
Apply wood glue to the supports and screw them to the inside of the rails, flush with the bottom edges and centered on each rail side to side. Use a generous number of screws to support heavy weight.
Screw the four rails together at the corners.
Place the seat on the supports and secure from all sides, top and bottom, with wood screws.
Construct the legs from plywood using four pieces for each leg, two 8 inches by 4 inches and two 9 1/2 inches by 4 inches. To be sure everything is square, level and lines up, make each leg separately.
Cut a 1 1/2-inch notch out of one corner of each of the longer pieces to allow for the drop of the support.
Glue and screw the legs together, positioning the notches against the supports. Cut 4-inch squares of plywood for the bottom of each leg. Use long screws from the outside through the rails and deep into the legs. For extra security, toenail using finishing nails from the outside into the legs on all sides. If you decide you want to put casters on the bottom of each leg, insert them according to manufacturer's directions on package.
Mix outdoor bonding putty and apply to every plywood seam. Let dry, sand and paint with exterior paint. Coat with polyurethane. Drill holes in the seat to allow for drainage.
Make two cushions for each lounge from 3-inch-thick foam, luan and water-resistant outdoor fabric by stapling the fabric around the foam to the back of the luan.