Chirs, wrap some steel around the outside of the magnet making a short tube, (stalloy or transformer silicon steel) a few layers is best. Cut some discs to cover the rear pole. Try to get a neat fit between the edge of the disc and the edge of the steel strap you've wrapped around the magnet.
Test the shielding on a CRT at a DISTANCE, move in slowly until you get a LITTLE distortion. Usually the distortion is worst from the rear of the speaker. Even manufactured shielded speakers will cause some distortion, (at about 2").
Walker is right up to a point; but you will also need another magnet glued to the back of the existing one in the 'repel' mode, that is where the magnets initially repel each other but then as you slip one over the other actually attract and clamp together in the centre. What this does is concentrate the magnetic flux in the gap, the little stray flux that is left over will be taken care of by the shielding cup as described by him.
I don't disagree with the comments about the bucking magnets. Good quality drivers tend to have better flux control and in the past the shielding has been adequate.
I wonder what effect the bucking magnets might have on the voice coil flux strength and distribution.
I suppose it depends on how close the drivers are to the screen. Most of the stray flux surrounds the rear of the drivers and is directed away from the screen anyway.
It won't hurt you to try shielding first and if it doesn’t do the job add the magnets. The magnets should live under the rear pole discs.