Never use a website to generate a password for something important like this. You can print out diceware passwords and roll dice.
You can just take a picture of a pile of dice, a pile of rice, or a tree, patch of grass, etc, and compute a secure hash/whatever and base six it to get the rolls.
> You can print out diceware passwords and roll dice.
Or on the CLI:
* https://packages.debian.org/search?keywords=diceware
* https://packages.debian.org/search?keywords=pwgen