The plug-in can be configured with a GPIO pin it then uses to turn power on/off and I selected pin 8. This meant I could solder up a very simple straight three pin header with 5V, GND and pin 8 next to each other.
There is a tiny gotcha with this choice in that pin 8 is GPIO14 which is used for the serial console on the Pi by default. So you have to use raspi-config and disable both the console and serial hardware in the 'Interfacing options' menu.
With that done, it was simple to configure the pin in PSU control and check it would turn the SSR on/off. This worked immediately but I found that the action was 'inverted' and the plug-in has a convenient tick-box that fixes that.
To manage the printer power I took the existing IEC mains lead and carefully stripped the outer sheath, exposing the individual wires. That made it easy to break the live connection and have it switched by the SSR without having joins in the earth and neutral.
So this was safe to have floating around on my bench, I designed a 3D printable enclosure and have published it on Thingiverse. This enclosure will obviously work for any use of these little SSRs and I'm tempted to buy a few more to have in stock for future projects. The cable channel is the right diameter for typical IEC leads and grips it when you tighten the cover down.