I spent all morning removing excess wood from the heel of the neck. First I laid out the semicircular shape I wanted. It is 25mm in diameter, slightly offset from the centreline.
Starting with small saws, I removed the most obvious excess at the outer corners. Then, using a combination of small chisel, knives, and sanding sticks, I got the vertical profile approximately right. The horizontal profile still needs some work.
A trick I used for the first time was to apply several layers of aluminium tape to the finished edges of the neck. This protected them from the sanding stick, and I ended up not doing any damage. The neck is already a bit narrow, so not having to clean up marks or realigning the edges will help maintain dimensions. Even so, this neck will be well suited to a smaller hand.