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Measure the 4 interior sides of your box and write the measurements down on a piece of paper so that you can easily refer to them later when you're cutting. You are going to build a railing for the base of your planter to rest flat, so the railing has to run from each interior edge of your legs to the adjacent interior edges. Your length should be 33 inches (84cm) and your width 10.5 inches (27cm) if you're building a medium-sized planter box. Using your 2in 2in (5.1cm 5.1cm) cedar boards, cut 2 longer pieces and 3 shorter pieces. Push your longer pieces in between the legs on the 2 longer sides, using the tension between the legs to hold them in place against the side. Push 2 smaller pieces in between the interior edges of the legs at both ends, and push 1 smaller piece in between the two sides in the middle, relying on tension to hold them still. You need the third piece in the middle to support the weight in the center of your box. Lay a level on top of each section of railing to make sure that they're flat. If they aren't, tap each section lightly with a mallet until they're level. With your pieces forced into place and level, screw your railings into place with 212in (6.4cm) wood screws placed 68 inches (1520cm) apart across each side of your railing. Hold each screw on the outside of your box, and place them so that they're level with the center of the railing. Drill a pilot hole using both hands to guide the screw into the correct location. Once the screw punctures the exterior, brace the inside of the railing with your nondominant hand and hold it in place while the screw goes all the way through the wood. A pilot hole refers to any small incision that you make with a drill head or screw to make the threading on the screw catch easily. To drill a pilot hole in your planter, simply take your drill head and pull the trigger for 1-2 seconds at the exact location where you want your screw to go. Brace the railing 612 inches (1530cm) away from the location that you're drilling so you don't drill into your hand. Make sure that you drill through the center of your center railing. If your center railing seems loose, you can use wood glue to provide some additional support.
Measure the interior sides of your box from leg to leg. Cut your pieces using your circular saw and fit them in by hand. Use wood screws to attach the railings to the frame.