[Closi]

> How do I lock up my cash in a similar way?

You buy a vault or a safe, which you can access with a 'pin-code' (in the fiat world this is called a combination lock). Safes come with two keys, which allows you to keep a backup of your 'secret' elsewhere, and you can also insure the cash inside if you want to pay for a full 'backup'.

In addition, "smart" safes and dual lock safes are available which have two keys, mean you need approval from another person to initiate the transfer.

It's not an exact 1:1, but you can hardly say that you don't have the option of storing Fiat by yourself.