Rip 5/4×6 decking in half and round over the edge with a block plane and sandpaper. Screw the vertical lattice to the side lintels and the back of the seat. Finish the lattice by screwing the horizontal lattice pieces to the inside face of the vertical pieces with 1-5/8 in. deck screws.
Sand the arbor to get rid of rough edges
‘Sand it?’ you say? Well, this isn’t a fine piece of furniture, but you will have a few rough edges and corners that could give you splinters. Just go around and examine the corners and edges and sand them smooth with 100-grit sandpaper. You may have to wait a week or more to sand if your treated wood is still moist because it will just gum up your paper. Once the project is dry to the touch, find an oil stain that suits your taste and brush it on. Have a rag or two handy to catch the drips and runs, and use dropcloths if you’re staining over a walkway. One coat should be sufficient, and you’ll need to recoat your arbor in about three years.