If I were doing it I'd ignore "clean" and go with wire and tape. I'd tape bits of wire so there's two ends under the button, and I'd do that for each button. I'd have a common positive wire for each button, and the other wire (one for each button) would have a different resistor on (not a mutliple of any other resistor either). I'd connect all the ends of those wires to one analog pin and ground, so it'd give me a different analog reading not only for every button pressed but for every combination of buttons pressed. In theory.