With a straight side piece, I finally got the shell assembled. I used thinner quarter-inch plywood for the back, reinforced with a 2” x 4” frame. This was one area where prior experience paid off. With the flexible nature of the thinner wood, I could compensate for very minor differences in length between the two sides by simply flexing the back a bit. As it turned out, when all was said and done my sides were 42 inches on the dot.