I had these washers split and drilled by a milling machine at Queens University Belfast’s friendly engineering department, but you can achieve similar results using a band saw and a drill press as follows. Alternatively, you can bypass the washers altogether and make ball-bearing contact switches more easily out of bent wire, or with screws or metal pins arranged in a triangle or square. See the files section for sketches.
Clamp and drill the washers in a drill press, centering the 2 small holes on opposite sides of each.
Use a band saw to cut each washer in half, perpendicular to the axis formed by the 2 holes. It’s easier if you clamp or screw the washer to a jig of scrap metal or wood.
File off any sharp edges. The washers will be exposed to fingers when installed on the instrument.