Step 4: Assembly
Use the armpit supports to triangulate the back and seat joint. Attach with 6"x3/8" lag bolts drilled into the end grain of the seat slats.
Take the piece that would have been the fourth slat in the back and notch it to interface with the underside of the seat. Screw it to the bottom of the third slat from the front in the seat as shown.
Sit the seat assembly down into the open "Ys" of the legs. The support on the underside of the third slat should slide down the front slanted face of the legs. Screw and glue liberally. Use what leftover lumber you have to add a cross-brace between the legs and two wedges on the back of the bottom of the seat. I also put in the two long bolts that had held the hand grips in place underneath the seat on the front side as a further brace.
If necessary, run a 3/8" dowel from one of the peg holes in the underside of the armpit supports back to the feet, as shown.
Break down, sand, and treat with your finish of choice if so desired.
All the waste generated is show in the last photo.