Unfortunately, it's not completely so. There is a lot of space around the small transfomers, that's for sure. The big transformer though... That is so close to the back wall of it's box that I could barely screw the box back after I applied 0.8 mm shield to the transformer. I have no idea why it is mounted there at all. Magnetic force changes exponentally with distance, so common logic dictates that it should be as far from the small ones as possible to avoid interference. Maybe I will relocate it, though it would mean utter disassembly and drilling a new mounting hole to the housing. Mu-metal is a great material, but it has a disadvantage: It provides very high permeability only if it's heat treated (annealing). Bending, or any mechanical shock distrupts the material's crystal structure. As a result, it's permeability decreases to the fraction of it's real potential. [Details here] I don't know if the same applies to Permalloy. If not, then that seems be a better choice. It's relative permeability is similar to mu-alloy, and it's also easier to acquire (at least where I live). It's expensive, but so is mu-metal. About the necessary length: 1 meter (~40 inches) length of 46 mm (~2 inches) wide strip is enough for two layers around the main transformer, one layer around each small transformer, and one layer of cover inside under the small transformers.