Rip a 3.5" inch board down the middle to get (2) 1.75" seat slats. Cut the length to 20".
Drill pilot holes and countersink on each end of the slat. The pilot hole should be with 3/4" of the slat's end. In practice, you can simply "eye it" so that you get as close to the edge as possible without allowing the countersink hole to ruin the board.