For encryption, the only way to be really secure is to have the same amount of randomness as there is data to encrypt, as a shared key. (A one time pad.) This isn't that hard to do so long as you can give it to the people that need it ahead of time, and should solve all kinds of theoretical problems. Mailing memory cards sealed in photoed glitter paint could work well too. It would be fun to 3D print a replica though.