Assuming that the driver is not vented and most tweeters aren't,all you need to do is give the magnetic field a low impedance path to travel through. A magnetic field would rather be in a piece of iron or mild steel than air. When you place a steel can over the magnet most of the magnetic field flows through the metal and not through the surrounding air. If you examine a shielded speaker you will find a cheap steel pot covering the magnetic assembly. In the old days horse shoe magnets had a bar of mild steel called a keeper which was kept on the poles of the magnet, this completed the magnetic circuit and helped the magnet retain it's strength as well as reducing the stray field. There should be no adverse effects from gluing a steel can over the magnet for the purpose of reducing the stray magnetic field,as this is what the factory does. If you are worried about the can resonating you can put a piece of Dynamat or No-Rez on it in addition to using silcone bath tub sealant for attaching it to the speaker.
Scotty
