This is fantastic. The one-handed organ keyboard is a work of genius, and it all feels like very simple organic technology: sets of tuned car horns are really cheap, and the rest is just a matter of wiring.
I'm severely tempted to buy a bunch of car horn parts and build my own MIDI-controllable one - every bit of this I can either buy on Amazon or 3D print.
No, they're not electrically powered, but given that you can buy an electrically-powered 5-horn kit from eBay for $20, it's certainly the cheapest way to go for a typical HN-reading hobbyist/hacker. Getting 14 horns is then $60, and the rest is relays a microcontroller, and a bit of 3D printing for the horns.
I'm severely tempted to buy a bunch of car horn parts and build my own MIDI-controllable one - every bit of this I can either buy on Amazon or 3D print.