I think aliexpress EMF paint has a good chance of working. All you need is some kind of conductor (usually graphite, that’s literally just pencil lead) mixed with some kind of adhesive. I’m not sure how the adhesive doesn’t stop electricity flow (maybe there’s just so much graphite it doesn’t matter), anyways you can buy graphite spray in any electronics hobby store, a paint is just that with a different consistency.

If you make a room’s walls and ceiling/floor electrically conductive (with paint or tin foil or wire mesh), you create a faraday cage. And as long as the gaps are small enough (proportional to the wavelength of the signal you want to block), nothing in those frequencies is getting in or out. A Faraday bag for your phone might be cheaper, though.