It needs to be ferrous (like iron) in order to deflect/route the magnetic field.
Try bending a flat piece of iron so that it forms a 4-sided box around your speaker. Drill holes in the diaphram side of the iron strip before you bend it around the speaker. This will divert the external magnetic field lines closer to the speaker, instead of letting them spray out farther where they can affect your reed switch. Try to use the thickest iron strip you can, to avoid having it saturate from the magnetic field so close to the speaker magnet.