It is very easy to forget that as soon as you add a metal enclosure you are not just wiring electrically, you are also wiring inductively.
Do you remember the old trigger operated soldering irons? They had transformer with lots of turns on one side and about 2 or 3 turns on the other shorted out by a thermocouple. The thermocouple received hardly any volts but massive current causing it to reach solder melting temperatures in a second or two and then miraculously stop on account of the fact that it was a thermo couple :)
Anyway the point is, you don't need a lot of turns to make a transformer, one will do. The easiest way to make a single turn transformer coil (earth loop) is to drill a hole in a piece of metal then put a wire through it. Pass an alternating current through that wire and current will start to flow around the rim of hole. Drill another hole nearby with another wire through it and you have a circuit. No nearby hole and it will merely start to warm up.
If you want to put wires through a metal plate I would suggest you knock one ruddy great hole, fill it with a Tufnol sheet then drill holes through that.
If you make sure that everything going in or out goes through the same hole it will all cancel out. Simples.