Stylophone Stylosette is a cassette-shaped instrument equipped with a single-octave touch-capacitive keyboard ... and the MicroKits Synth-a-Sette, a cassette-shaped mini-synth released in 2023 ...
complete with a full-sized keyboard spanning two octaves and a MIDI interface. Korg Introduced the Monotron analog mini-synthesizer back in 2010. They also dropped the schematics for the synth.