Print it on a piece of paper and put it in a lock box.
Yep, this is the way. It survives human memory and doesn't depend on software.
If you keep it in a dark environment that's not super humid the ink should last a really long time. Even in non-optimal conditions (NY summers with high humidity, etc.) I've had regular pen ink last for decades with no signs of fading away.
Better still: LUKS allows you to set up multiple entry keys, so use two, either of which will grant access to the drive.
* Your preferred memorized passphrase and will never be written down anywhere.
* A random key you can print and store in a box somewhere.
Then if your backup paper gets lost, you can revoke/replace it without having to abandoned your memorized favorite.