Won't "Mu Metal" work? Also I have heard In High EMF areas the cage should only be 3 sided otherwise you risk trapping frequencies in your cage and increasing exposure.
This is true in setups that have leaks. For ex, the diy pics from above most likely have leaks. the same goes with netting which have a curtain opening, or ones that use materials like Velcro. If you can't get a fully sealed setup then having a 5/6 or 4/5 setup may be the safer option in high EMF zones. I think this is more the case when you are very close to things like cell towers or your bed backs onto your fridge, a high voltage electrical panel, etc.