sci.physics.electromag

From what i've read, it sounds like the permeability
of a conductive enclosure will increase the density
of the lines of magnetic flux within the conductor,
so that the magnetic field within the enclosure
will be less intense, which in effect can make
the compass less accurate (or slower to respond).

I'll be using a Honeywell digital compass, and
I need to know if the metal chassis will need
to have openings around the compass or not.

Or, i may need to use a plastic enclosure instead.

Both purely mathematical/theoretical and real-world
answers welcome.