If you are planning to store it in some external storage like a USB flash drive or memory card make sure you do it on a malware safe computer and/or turning off the internet connection is also a good additional safety measure.
Going for hierarchical deterministic wallet will be better that can generate seed phrase which can be written on paper or a metal sheet, it is better than backups on USB flash or memory card, the reason is because people can create an offline wallet but the way they backup using USB flash and memory card can expose their seed phrase or keys online. But paper backup can be difficult for private key back, that is the reason I will go for seed phrase means of generating private key which are easy to backup.
The idea is to use hardware wallet to create 24 word pass code with additional 25th word password but then secure those codes inside of safety deposit box in bank. The hardware wallet itself does not have any value to anyone after the wallet has been created, right? Because it does not hold the passwords.
The additional word is called the passphrase, the passphrase is not stored on the hardware wallet, but I will say trustworthy hardware wallets like Trezor and Ledger Nano. If an hacker got hold of such wallet, he will need to brute force the passphrase before he can be able to access the wallet and this can be very impossible if strong passphrase is used.