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Mark off a rectangle (7 feet by 9 feet, 10 inches), then use the shovel to remove the grass and 1 inch of the topsoil. The sandbox, which has final outside dimensions of 6 feet, 8 inches by 9 feet, 6 1/2 inches, will be held in place by Mother Earth. If you need drainage, dig down 2 more inches and put down some gravel. Make sure the ground is level by laying down a board and checking it with a carpenter's level.
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Set the two external frame boards into the dirt with the notch cuts at one end (this will be the lid storage end). Set the toy box end-piece in place at the other end. (The end-piece needs to be 1/2 inch lower than the internal and external frame boards. Don't cut it to make it lower; just dig it 1/2 inch deeper in the ground.) Square the frame using the carpenter's square and check that it's level. Screw the toy box end of the frame together using 3-inch screws. Set the internal frame boards in place, leaving 8 inches in between each one and the external frame boards. Square them, then screw to the end-piece with 3-inch screws.
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Using a rubber mallet or hammer, tap the two internal frame ends in place - one at the toy box end, 19 inches in from the toy box end-piece, and one at the lid storage end, flush with the L-shaped cut in the internal frame board. Lower these boards by digging them into the dirt an extra 1/2 inch. Screw in place with 3-inch screws.
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Place the lid storage end-piece into the open end of the sandbox and fasten with the 3-inch screws. Tap the two blocks into place at the lid storage end to keep raccoons and kids from crawling underneath. These boards sit 1 inch in from the L-shaped cut in the internal frame boards and are set into the dirt 1/2 inch lower. Screw in with 3-inch screws. Double check the levels and squareness of the box. Measure across the two diagonals between the far corners. If your box is square, the diagonals should be equal. Tap the frame with the mallet to make changes. To level the box, pry up the boards, push a few handfuls of dirt underneath and tamp the boards down.
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Place the two seats down and screw in place with 3-inch screws.
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Assemble the toy box cover in what's called board-and-batten style. Line up the four toy box cover boards and lay three toy box battens across them. Place a batten 2 inches from each end of the boards and one in the center. Screw the battens on with 1 1/2-inch screws, holding the boards together as you go. Screw the strap hinges onto the battens of the toy box cover. The hinge should open with the pin on the outside of the box. Chisel three 1/8-inch-deep by 3-inch-wide notches in the toy box end board to match the hinge spacing. Have two helpers hold the lid up, and screw the other half of the hinge on the box.
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To build the seat for the lid storage end of the sandbox, again use board-and-batten style, with the four lid-end seat pieces as "boards" and the remaining four lid battens as "battens." This time, screw the seat together so that two of the battens are on the very ends and the other two are spaced evenly. Screw the seat onto the frame with 1 1/2-inch screws.
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If desired, apply urethane or paint to the exposed surfaces. Put the tarp in the bottom of the box and poke 50 drainage holes in it (this keeps the dirt and sand from mixing and lets water drain). Fill the box with sand and choose a lid design.
