I like the survivability, and the constraint to be doable with common hand tools.

For more information density, and easier readability by a less-technical person who inherits it -- at the cost of requiring special tools -- I wonder about using number&letter stamps with a hammer.

Or, if you permit very special tools, laser-cut alphanumerics (base16, base58, or base64, for arbitrary bits; or alphabetic passphrases). Either engraved, or cut fully through, like an old drafting lettering stencil.