Relay box for use with Octoprint

I'm using Octoprint on a Raspberry Pi and wanted a way to switch my printer on/off remotely, because I work in a different building to the printer. The printer can now be preheating whilst I walk over or I can switch it off if it's finished printing and it'll be a while before I get over there or I want to collect my print but leave the cooling fan running for a bit. Plus I fancied a fun project ;)

This box sits between the mains power and the printer PSU. Internally, the live mains cable goe