This now has a custom PCB specifically designed for the Atari ST.
When I first built these, I used one of my generic USB keyboard controller boards, with a short cable to connect to the STs keyboard cable.
The cable from the keyboard plugs directly onto the connector on the PCB, and a USB cable into the other end. This should now be a neater solution.
The controller can be mounted at the rear of the case, so the USB cable can be plugged in externally.
Alternatively, it can be mounted on the back of the keyboard (or anywhere else inside the case), if you want a captive USB cable, or plan to install a Raspberry Pi or other small computer inside the case. (see a previous project shoe-horning an intel Nuc inside an Atari ST).
The keyboard incorporates two 9 way D joystick ports, accessible underneath the ST. These are enable and work as USB game controllers, along side the USB keyboard.
This can used used anywhere a normal USB keyboard can be used, with a PC or a Raspberry Pi etc. The keyboard is fully mapped in the controller firmware, so just plug in and you can start typing. Most of the keys map to keys on a modern PC keyboard, those that don't are mapped to something appropriate (Help is set as a Windows key as there isn't one already, and Undo is mapped as Control Z).