There’s always 5D Optical Data Storage, which might store data in glass for a long, long time. (It’s not really 5D; it’s composed of discrete nanostructures which have five attributes.) You would only have to write to a new disc every N billions of years.
Maybe try using a file to make flat sides on the outside of the head so you can use a wrench on it? (Not sure if that would be better or worse than the Dremel/slot idea… you would risk scratching the surface of the object it’s on.)