If your looking for a insulator to block magnetic field, try lead, bakeralite, Heavy plastic or glass.
No known material will block a magnetic field. Many magnetic materials will concentrate or redirect the field (such as shields used in electronic devises) but none will actually block the field.
Lead has been the standard answer for radiation for years. Mu metal has been used to shield electronic components from adjacent radiation emitting components. Shielded copper is normally provided in cables to prevent general electrical interference.
