I decided that I wanted 1) to bend the 1/8 ply parallel to the face grain, and 2) Didn't want a seam to fall on the tightest radius, which is 18 on outside, so 16.5 on the inside. So I have this gap to fill. Rather than stress about how to attache the ceiling I just glued it up and shot some pneumatic staples in there. Seems tight. I will of course be committed to some trim work to cover the staples.