It is also cheaper than metal boxes. Another source is the thick aluminium foil used for what we used to call TV dinners, massive amounts of which just seems to get thrown away. Use some plastic foam to keep your circuit board away from the metal, but do make a good contact between the foil lining and earth and a foil lining stuck inside the lid. The electrical field pick-up by the new loop is still there, but I have found that at times it disappears completely, furthermore I have realised it is the same noise that makes my vertical practically useless as a receive antenna. So the source is most probably a light dimmer or something similar nearby.
Solution build a noise canceller.
The KI0LE loop is very sensitive but subject to local noise pick-up should be good out in the countryside though. I have also been trying it out on VLF with a couple of Rennato Romero s designs for sensitive audio pre-amps and getting superb results. At these frequencies this type of construction is really coming into its own. The purpose of the pre-amp is partly to get a bit of gain, but primarily to tune the loop and match it to 50 ohm coax. My idea is not to tune the loop, but simply transform the complex impedance.