I've read and been told that to properly shield a loudspeaker, the process is done at the driver and not so much at the cabinet or area between the magnet and the tv picture tube. Thus said, the driver(s) have another magnet coupled to the voice coil magnet (designed) where they cancel each others stray magnetic flux. I'm sure foil and other means help but a tv tube is sensitive to magnetic fields and if you try your idea (why not) be sure to keep a close watch on it, fore you can 'magnetize' the tv picture to where it shows a 'purple-pink' hue to an area of the screen, then you have to degauss the tube. Tv's have circuits that are designed to counter act these fields, but not a huge force field. Good luck and experiment..type back what you find. Oh yes, using a ferrous type material (like a magnet will 'stick' to is not a good idea, it too will become magnetized).