Build bed upside down. Set a 4-foot 2-by-6 on its thin edge on pavement, and place a 16-inch post at one end. Secure post with two 3½-inch screws. Repeat at other end of board. Repeat with other short board.
Join short sides with an 8-foot board; and secure with two screws. Add other long side. Add second layer of 2-by-6s.