I asked if someone could create 3D printable JAMMA key, and Greg Collins designed one. JAMMA keys prevent arcade PCBs from being connected with reverse polarity, which could kill the PCB. For some reason, the tiny pieces of plastic were hard to come by and expensive. Now they can be printed for pennies.
The print is available for free at Thingverse as a single key, or a sheet of 50.
A big thank-you to John (apdwaldo6) for printing me off a bunch.