[Dix Dogfight]

If want to shield the sorrundings from a magnetic source you have to create an easier path for the magnetic flux than the air.

The way to do that is to create a container made of iron or nickel. You don't have to make it spherical or any other tricky form an ordinary box will do the trick. And the strenth of the magnetic field determines the thickness of the material used however you probably don't have to make it thicker than 2-3mm.
Just make sure the "box" is closed on all sides, so u don't leave an open top like the do in the link that Komic posted.

Grounding the "box" isn't neccesery, since the magnetic field is alternating. However a grounding prevents any form of static electricity to collect on your "box" wich is BAD in a computer. Just make sure u connect the "box" at ONE place with a wire to the case of your computer. Making more than one connection can create alternating corrents in the computers case an that is also BAD.

Copper and Aluminum have the same magnetic permeability as air which makes them WORTHLESS as materials in this application.

Wow that was a long post. hope u have time to read it.