The most famous modular synth DIY project in Japan (Patreon)

It's called Analog 2.0.

 It's not a eurorack, but it's a project that covers basic analog modules.

https://github.com/naokiiwakami/analog2.0/wiki

You can create an old system for less than $200 with a circuit made up of easily available components.

You can even buy PCBs if you pay some money.

This is also a project I worked on when I was a fledgling engineer.