I have engraved private keys onto titanium plates for exactly this purpose. They are kept in a box with the other valuables, like a hoard of POGs, various tamagochi, Princess Di beanie baby, and some very rare and powerful Magic: The Gathering cards. Dang kids are gonna be floored when they find this box next to my partially decomposed earthly remains.
lol, full bullet proof . I b t it won't get stolen. Congratulations for having such a brilliant idea and future for generations after you.