When I finished cutting the boards I realized two things: I needed to use a jointer first because none of the boards were straight laterally, and I could have cut them the same as last time, but a quarter-inch long, and trimmed the ends as necessary.
I put a nail in each end to see how I did. That lateral twist came back to haunt me. That bottom board is flush against the window, but an eighth of an inch away on both ends
Also, there are little things stuck to the frame, making the boards fit worse. I am sure that a sanding block would remedy that, but it is the smallest problem here.
I know what route I need to take!