1. Place the two arches 4 feet apart, as shown, in the desired location. Unscrew and remove the seven existing crosspieces atop each arch.
2. For normal support, attach seven 1-by-2s to the arches with 1¼-inch screws, using the existing screw holes. For extra support, attach six additional 1-by-2s between the other seven.
3. Saw the lattice panel in half crosswise. To cover exposed edges, attach lattice caps with two 1¼-inch screws per cap.
4. Across the back of one arch, hold one of the capped lattice panels at the same level as the existing lattice panels, mark the corner edges with pencil, and set the panel aside. At each of the marked corners, attach an L bracket with four 1¼-inch screws. Slide the lattice panel into the L brackets and attach to the sides of the arch with eight 1¼-inch screws. Repeat for other arch.
5. Place seating planks on built-in crosspieces.
6. For extra support, attach one 2-by-4 horizontally to the back of each arch (just behind the seating plank) with four 2½-inch screws.