[mathgorges]

Banks’ safe deposit boxes are a good choice if there are any near you, though they’re often expensive.

If you have a lawyer/attorney that you trust they’re often in the habit of securely storing physical documents, but again, expensive.

Personally, I (a cheap-ass) keep a copy in a wooden box buried underground in a secure location along with a few other things.

However, the security of physical things remains a difficult problem that is most easily solved with money!