Came across this linked from computer security expert Bruce Schneier’s blog: how did people in pre-industrial times protect paper letters from prying eyes? There were no mass-produced envelopes in those days, so they resorted to “letterlocking” -- intricate ways of folding and sealing a letter to form its own envelope https://www.atlasobscura.com/articles/what-did-people-do-before-envelopes-letterlocking.

And yes, there is a whole website devoted to this: http://letterlocking.org/.