Measure and cut two 2-inch by 6-inch beams to 10-feet long with a circular saw. Cut eight 2-inch by 6-inch beams to 9-feet and 9-inches in length.
Assemble the basic frame with the 10-foot beams running horizontallly and the 9-foot 9-inch beams running vertically between the two 10-foot beams.
Attach the first and last 9-foot 9-inch beams so that the edges of the beams are flush with the ends of the 10-foot beams and the 6-inch sides stand straight up for all the beams. Drive two 3-inch, self-tapping, exterior grade wood screws through the exterior edge of the 10-foot beams into the ends of the shorter beams with a drill and screwdriver drill bit, creating a basic box.
Make a vertical line mark every 16 inches from one end of the box on each of the 10-foot beams, creating pairs of marks. The result will be 7 pairs of marks with the last pair sitting 8-inches from the end.
Attach the 9-foot 9-inch beams to the pairs of marks with two 3-inch screws drilled into each end to complete the frame.
Lay out sheets of plywood so that the ends of the plywood sit on top of vertical beams. Eight-foot sheets will be the best fit working from one side. Cut additional sheets with a circular saw to 2-feet to fit the remaining exposed section.
Attach the plywood sheets to the frame with 3-inch screws spaced 8 inches along each vertical and horizontal beam.