Popped off seatback assembly and discarded clampy thingies. Trimmed all.
Made a jig to hold the seatback assembly at the desired angle. Filled remainder of upper seatback flox corner, prepped foam and applied 1 x BID, overlapping the seatback a generous 1". Maybe 2... Peel plied join. I used Colan BID just one more time, and I don't like it. It doesn't drape or wet out as well and unravells badly at the edge. I resolve to only use it where there is not an exposed edge or complicated transition.