Little red cards with one time codes imprinted, folded in half then sealed in Perspex with a breakpoint. I may have been distracted with old school launch codes there....
Or perhaps a constantly changing keys e.g. RSA or google token generator style... This avoids accidental re-use of the same keys... Interestingly would also be a separate "component" not directly involved in the encryption process. Who knows, this may even take you out of scope.
Or simply confuse them by making all your comms in plaintext - they are probably not even looking at plaintext - just agree some codenames for your mates...Mr Pink, Mr Black, Mr Brown etc :)