My first "cheat"

I've decided not to taper the back slats. It was just going to be far
too complicated and take too long to do. So I'm removing one of the
panels and simply fan the slats out. Should work, although it won't
look as good. Bit of a bummer but I have to accept some compromises...

Next stage, cut the tops of the back slats in a fan shape and it'll
slowly look like a proper adirondack / cape cod / muskoka chair.