Maybe you have a bunch of old Apple computers lying around your house? Why not hack your Apple II Plus computer as a USB keyboard?
Connecting an Apple II keyboard to a computer with USB is surprisingly easy with a Teensy board. Arduino (Teensyduino) code is available at https://github.com/afiler/keyduino . The Apple II uses an ASCII keyboard, which means that rather than returning scan codes, it returns a 7-bit ASCII value. This also means you can’t read the state of modifier keys like shift or control independently.