You take an old speaker you don't want, and then cut off the frame using a hacksaw. Hopefully the scrap magnet is smaller yet almost the same size as the good speaker.
The only problem is attaching the bucking magnet, since the 2 will want to repel each other like there is no tomorrow. And you don't want to be putting forces on the speaker's frame, you could distort it. I imagine a good layer of epoxy could do the job.
Something I've always want to try is this. Transformers use a magnet friendly material, ferrite. Take a Xformer apart and you'll see it uses several plates of ferrite to make the core. Get that ferrite core out, and now split the layers apart. The whole thing is likely enameled over, so it won't be fun, or easy. Once you're done, you'll get at least a square foot of 'shielding material', since the ferrite shunts the magnetic field onto itself. It sounds great in theory, but I just haven't had a need to try this yet.