EMC shielding - Materials to use / not to use

Can any here help me answering what type of materials to use and NOT to use, when making an EMC tight enclosure around a smaller box (such as 15cm * 30cm)?

I hear, that using stainless steel is at no use in the lower frequency area (magnetic range). I especially ask about this type of material, because I have seen some workshop making a box of this material. I tried to put a magnet on the side of thihave some s box; the magnet fell down. I think I have read something about, that the steel should contain carbon, to be usefull for stopping lower frequency interference.

I would also be glad to have some references covering EMC problems and design.

Thanks in advance