First, Russian DIYer Dmitry Morozov, better known as vtol, has a website full of his colorful instruments. Nice panel art! He apparently makes limited issues of some of them for sale.
Then, Flickr user “jugger-naut” built a tube synthesizer in plug-in module form.
And Joe Paradiso built a hybrid synthesizer into a CAMAC crate, to be controlled by a PDP-11 minicomputer, in December 1979. Don’t throw it away, Joe, it’s a priceless historical artifact!
Mike Zee is a musician and prolific DIY builder in Poughkeepsie, NY. (That’s like saying Sean Connery is an “actor of some repute in Scotland”, I suppose.)
His main site for custom work is here. His cabinetwork is so beautiful, it will make you cry. Be warned, you will be exploring every link there. He has schematics of almost everything he’s built — clever designs, easy to reproduce.
He has a Soundclick page if you want to hear samples of his music. And he has a YouTube channel.