[adrian_b]

You can store it encrypted with a password on a USB memory that you insert when booting the computer, like you would use a key for starting a car.

This is what I actually do. I also store the OS kernel on the USB memory and I boot from there, with the root file system set to mount an internal SSD. The SSDs in the computer are completely encrypted with such long "super secure" keys (distinct for each SSD and selected automatically based on their serial number), and they do not store any information about the keys.

I have used this system for years and I find it very convenient. My computers cannot be booted without inserting the bootable USB memory and also giving the correct password. I have multiple bootable keys stored in different places, for the case when one becomes defective or lost.