Lay another one-by-three batten diagonally across the opposing ends of the two battens just secured to the back of the door. Measure and cut it as a diagonal brace with mitered ends to fit between the two battens at a 45-degree angle. Secure the brace with rust-resistant screws. The brace will prevent the doors from sagging and provide additional rigidity. Assemble the other door in the same manner. For symmetry, angle the brace on the second door in the opposite direction. Construct the lid in a similar manner.
