A small passive autofire board that I designed, based on a board shared by @emphatic on ArcadeOtaku over 10 years ago. There's also a schematic on the ArcadeOtaku wiki made by @yosai which I double checked against, however, I used the same components as on the original board.
A small addition I included is an ON-OFF switch.
Basically how the autofire works is that you connect the board inline between a ground wire and up to two buttons that you want the autofire on. I've tested a handful of boards with it and it seems to works well enough. YMMV.
Gerbers attached below. The zip also includes a BOM (alternatively, you can use this Digikey link).
If you order from JLCPCB, there's a placeholder text under the 74HC04 for their silly batch number.
(PS. I have a handful of populated boards with wires that don't include a ON-OFF switch (based a previous design I made). If you would like one, I can sell these for 20 euros including postage)