That little piece of ebony took 2.5 hours to get into that shape! And it's not finished.
Working with a combination of hacksaw, file, and sanding stick, I got the nut into the right external dimensions. Rather than leave the ends perpendicular, I made them slightly flared toward the scroll, to match the outline of the pegbox at that point. I lightly glued the nut onto the fingerboard and left it clamped.
Now that the nut is attached to the fingerboard, it will be easy to finish the profile and clean up each surface. Once the fingerboard is lightly attached to the neck I will match the edges exactly.