Kasbah posted his Nomech mini project, a 4×4 projected capactive touch board using Atmega32u4 and back-fire LEDs for each button in the project log forum:
It fits SoB 100x80mm but the top acrylic will sit right on top of the PCB as the dielectric properties of whatever is touching the electrodes matters a lot.
If this works well then I might scale it up to 8×8. I want it to work as a cheap monome replacement without using mechanical buttons; hence the name “nomech”.