Determine the desired size of the sandbox. Cut the four sides from 2" x 10" boards. Drill pilot holes at each corner and fasten boards with 3" galvanized decking screws. To prevent the boards from warping outward, make sure the natural curve of each board faces into the sandbox.
Cut two pieces of pressure-treated plywood that match the outside dimensions of the sandbox. Attach one of the pieces to the bottom of the box with 3” screws. Reserve the other piece to be used as a sandbox cover.
Drill two holes 6" apart on both sides of the plywood cover. String a length of marine-grade rope through each set of holes, knotting them on the underside to create handles. Fill with kid-safe sand.