Saw down three 1-by-6-inch boards to a total length of 14 inches.
Cut a shallow arch out of two of the boards with a saw, leaving approximately 1/3 of an inch at the arch's deepest point. The arch should cut into the board's side, not its face.
Sand the boards with sandpaper until they are smooth, putting a slight round on the edges.
Fix the boards together using wood glue, placed 1/2 of an inch from the edges and across the middle. One arch should face downward and the other upward, with the uncut board in between.
Screw the boards together with a screwdriver by placing two 1/2-inch screws about 1/2 of an inch from each edge of the seat.