Surely the ultimate solution would be Scandoubler for the Amiga, RF to VGA box for the C64, and a console switch so as to use just one monitor.
Anyway, I'd guess the best would be ferrous material - ie the most maliable sheet steel you can lay your hands on - lead might be overkill. Welding sheets are big and cheap (and quite soft steel), could try that doubled over (easy to cut with tin snips).
I believe computer speakers use mild steel sheets, which 'conduct' the magnetic fields, and thus diffuse them simply because of their shape (ie. bigger than the source of the field, and flat)..
