Wire multiple protective earth conductors together
Using a plastic enclosure is problematic for mains grounding and safety, so I say use a metal enclosure.  Ground the metal enclosure.  Use standard knockouts for your entry and exit of AC mains cable or conduit, with correct cable clamps. Everything done with mains must be to Electrical Code rules, which are 90% about installation methods and packaging. 

Some enclosures even have DIN rail built right in.  

You can get appropriate metal enclosures at an electrical supply house. If they have a warehouse full of junction boxes, conduit, and spools of wire, that's the place.

If their enclosure doesn't have knockouts in all the right places, they can sell you a drill or punch that'll make standard sized knockout holes. 

For small stuff, a 4" or 120mm square box is very handy.   

Lastly you don't need to have one grand big box that has all the mains switching, it's a lot easier to run a thermostat wire and bring the low voltage to the mains than the other way 'round.   So feel free to have several smaller boxes in convenient locations if that makes sense.