Dig six holes in the ground, in two rows of three, so that the six holes make a rectangular frame that is 3-by-6-feet. Make each hole an 8-inch-by-8-inch hole 16 inches deep. Cover the bottom of the hole with 4 inches of gravel.
Position a 4-by-4-by-96-inch treated post within each hole and pour cement into the hole around each post. Hold the posts in place until the cement dries. Let the cement dry for six hours.
Set a 4-by-4-by-72-inch post across each set of three posts so that it is evenly spaced across them. Position a 6-inch galvanized metal L-bracket between the horizontal and vertical posts. Drill a 3-inch galvanized wood screw into each hole in the brackets.
Position six 2-by-4-by-36-inch treated boards on top of the horizontal beams. Space them evenly along the top of the beams with their long, thin sides facing downward.
Hold a 3-inch metal L-bracket against each of these 36-inch boards and each horizontal beam that it connects to. Drill 3-inch wood screws into each hole in the brackets.