Reader Ville Aho created a DIY Ondes Martenot style synthesizer, the MonotrOndes, based on a Korg Monotron.
I built this Ondes Martenot-style musical instrument. It’s quite cheap to build.
The sound generator is Korg Monotron. The most expensive part besides the Monotron was a multiturn 100k potentiometer.
The demo video features multi-tracked tracks of the MonotrOndes.