I'm working on data diodes... I've got the parts, now I've got to gather enough spoons[1] to do the work to make them useful.
Details: Building a physically irreversible way to transmit data in only one direction should be easy, using a 6N137 optoisolator. The hard part is the proxy servers that you put on both sides of it. Old timey Raspberry Pi B+ boards are my current choice for hardware, as they have NO wireless capabilities, and do have reasonable ethernet ports.
I want to keep the retail parts cost under $100, and open source it all.