I designed this model in order to create a rack-mounted female to female Speakon adapter, however the design not only would be compatible with any Neutrik-style chassis socket but also is able to be used by itself outside of the rack.

It uses 2x M3 14mm screws and 2x M3 nuts per side, and the same piece is printed twice for the top and bottom, so 4 of each total. Screws between 12-15mm may still work depending on your application. It also my be worth investigating screw types; as you can see in my last pic the screws I used only just allow for clearance for the lock for the connectors so a flat head may have been a better choice.

I printed using eSun PLA+ filament on a PIP with default optimal settings (w/ tree supports), feel free to experiment with other materials!

Also make sure you remember to invert your wiring correctly, unlike me on my first attempt.