Paranoid overkill really. Suppose you didn't have access to a locked lead box and stored your extremely large sum in an envelope, you would feel silly in 10 years when you realized someone was able to steam the envelope and use the raised letters from the pressure when writing to reveal the key without removing the tamper proof seal.

There are better, more modern methods of applying a seal or printing but it increases the attack surface so for the truly paranoid, they should probably be avoided (unless using something like Shamirs Secret Sharing Algorithm).