> Haven't found a low-tech way to make a piezo earphone just yet

The closer way I've found is this guy making it with a recycled transformer, a hard drive magnet, a jar lid, some wood, and some cables.

https://www.youtube.com/watch?v=cEMXlLqwwTA (First part. 7:07 min.)

https://www.youtube.com/watch?v=MXpprO5h79I (Second part. 9:32 min.)

Those videos are in Spanish but subtitles are available.