How do you shield a Center speaker?

I have to agree with Merganser.

It's difficult to "shield" magnetic fields.

The bucking magnet solution is probably best. From outside
the speaker, the magnetic fields of the two magnets - the
bucking magnet and the one in the speaker driver - will
essentially cancel.

Inside, the voice coil is much closer to the driver magnet -
so it doesn't affect the driver much.

Putting a shield material around the speaker would be more
of a sonic degradation than any effect from the bucking
magnet. Additionally, one can't put shielding material
in front of the driver - so the speaker will leak magnetic
flux through the front.

Dr. Gregory Greenman
Physicist