Banks can accidentally lose safe deposit boxes

1. Buy private safe deposit box insurance. I won't link it to avoid impropriety, but easily found via Google.

2. Have joint ownership, so that death isn't an issue.

3. Keep all your items in a dry bag if the bank is in a flood-prone area.

4. Assess the probability of the bank emptying your box (nearly zero) vs a safe in your house being stolen.

5. Digitally scan irreplaceable original documents, so you at least have a copy.

6. Print out a high quality document with a fake letterhead of your favorite 3-letter agency, informing the opener that they are committing a federal crime and will be personally liable. Maybe claim that the bag (See #3) is a diplomatic pouch and opening it is an international incident. Or other stern warning. Print a phone number on it to call immediately. Also put a fake gun in the box to give an air of danger. YMMV.