John in NM
Spindle sander and frequent testing, like a lot of other operations you just have to sneak up on it. It was tempting to pattern rout it, but I figured it wasn't worth making the fixture for a one off.

Easy to do with a rasp too, probably easier than the sander - spindle sanders give you the illusion of automatically square and with a rasp one knows to be careful about keeping it square.

The old trick for fitting is to wipe something on the metal that will transfer to the high spots on the wood when dry fit. Black artist oil paint for example, though white works better on walnut.