There’s got to be ways of hiding it in plain sight that’s low risk of people noticing, and you misplacing the single copy. I’d say at LEAST use a passphrase. Also, what if you also used a child seed (that only you know) with a second key?

So if they find it in the book they’d still need to know passphrase 1, the child seed and the second passphrase… and the fact that you’re using this method.

But seend some sats to the key found in the book as a decoy.

Single key but multisig. 🤔