I filled the space between the spreaders at the top and bottom of the windows with 3/4-inch rigid insulation after the windows were set in place. I also placed 1 1/2-by-2-inch rigid insulation to fill voids where the Roxul insulation had been held back.
On each flared side, I tacked angle-ripped 3/4-inch rigid insulation in place, keeping it back from the plywood bucks.
I installed 1 1/2-by-2-inch nailers to the front edge of the sill to provide a good screw base for the interior drywall.
A scrap block pushed the tape into the intersections and smoothed out possible wrinkles.
I laid templates, which I had made with 12-inch scraps, on the flared left and right sides of the sill and measured between them.
I transferred the angled scraps to a piece of Baltic birch, separating them by the measured distance.
The nosing overhangs the flared sides 1 1/2 inches on each side. I cut the maple to length, then placed it against the Baltic birch, mindful to maintain the overhang, and drew registration marks that would allow me to align plate-joiner cuts.
To cut slots for the biscuits, I adapted the fence to my plate joiner with a piece of 1/4-inch plywood to get the depth of cut I needed.
Here is the finished window.