This will be the companion to the NSA-Away (https://hackaday.io/project/1569-nsa-away).

The more primitive the means of one-way data transfer, the more secure, but the less convenient...

The general idea is that you have an air gapped computer or microprocessor device (think RasPi) that encrypts the plaintext with a one time pad (obtained from the NSA-away or similar pad-sharing device). Then, a laser transmits the cyphertext to a photosensor, from the air-gapped device to the networked device. After that the networked device can be used to send the ciphertext across the network. Finally, the same optical method is used to transmit the cypertext from the receiving networked computer to an offline computer that decrypts the code using the second copy of the one time pad. If this is done with a new, perfect-random pad that's the same length as the plaintext, it might be unbreakable.