The PiCap from Bare Conductive uses conductive ink to transfer touch impulses to the Raspberry Pi. They’ve written a lovely little tutorial on how to do it yourself and have put the code on GitHub. If you want to do it “properly”, you’ll need to follow their screen printing tutorial but you can get similar results by just painting the note ‘buttons’ on a piece of paper or card. The code is based on the FluidSynth synthesizer which uses sound fonts and is the basis for my own music tech project, the Music Box. Take a look at the tutorial here.