Just had a duh moment. I’ve been trying to think of how to make a socket wrench style adapter to screw into the end of a servo. This entire time I was looking at it like it would attach to a servo horn. This was an absurd concept seeing as this would mean I was building an adapter for an adapter.
Take a socket wrench (or better yet, one of those adapter to adapter things), cut off the end I want, drill a hole in the non business end, drill a hole for a locking pin. Thread locking pin hole. Add (striations?) to main hole.
Profit.
Really, I should be casting them, but that gets back into a get area of diminishing returns.
Update from Nod: I could meet the two ideas in the middle; I could fab in wood.