Set two 28-inch boards on edge, parallel and 18 inches apart. Set two 18-inch boards between them at right angles, spaced 27 ½ inches apart. Screw through the 28-inch boards, into the 18-inch boards. Use eight screws for this task. This is the frame to your pochade box.
Set one sheet of plywood on top of the box frame so the edges are flush. Screw through the plywood and into the frame using two screws for each edge. This is the bottom to your box.
Drill a 1/4-inch hole through the center of each 17-inch board. The center of the hole is 8 ½ inches away from each edge and 1 ¾ inches away from each edge. Lay both of these boards on the last sheet of plywood so they're spaced 18 ½ inches apart. Position these boards so their outside edges are 1 ½ inch away from the 19 1/2-inch edges of the plywood, and so their ends are each ½ inch away from the 28-inch edges. Drill a 1/4-inch hole through the plywood using the holes in the boards as a guide.
Screw two half-hinges to the edge one of the 28-inch boards on the frame. Each hinge should be 5 inches away from the end of the board and should be able to fold 270-degrees around the outside of the board as well as the outside of the pochade box frame.
Screw two screw hooks to the lid so they are each ¾ inches away from the 19 1/2-inch edges and 1 inch away from the 28-inch edge that is opposite the side with the hinges. Screw the last two hinges to the 28-inch board that is opposite the side with the hinges. Position these pair of screws so they're ¾ inches away from the ends of the board and 1/4-inch away from the top edge. Open your lid and secure your chains to the hooks.
Open the lid and place the 17-inch boards on the lid so the holes are aligned. Fasten the boards to the lid using the bolts and wing nuts. These boards will hold your drawing paper or canvas.
Close your lid and screw the flap portion of the latch to the top of the plywood so it can fold over the 28-inch board. Center it using your tape measure so the distance on both sides of the latch to the ends of the 28-inch board are equal. Screw the other piece of the latch to the 28-inch board so when the latch closes you can put a lock on it to keep it closed.