Super fast discharge/spark generates harmonics and ringing in a non-resistor plug. The resistance softens the discharge (slower di/dt) and thus no radio interference to speak of. Metal to metal rubbing is the culprit, not the ignition itself.

Some time ago I spent half a day looking for radio interference sources. I used an oscillator to trigger the CDI in lieu of a hall sensor on our engine. I had the spark plug laying on the bench sparking away. With the 'antenna' near enough to almost touch the spark itself or anywhere near the CDI board, I saw zero, nil, nata, nothing that would register on my digital phosphor scope. The 'receiver' was a germanium diode/LC tank. However, when I rubbed/scratched a screwdriver tip on something metal nearby I would pick up beaucoup noise even feet away from the antenna.

I mentioned the copper paint just because someone asked. In other words, I do not believe shielding any box will buy anyone anything in our gassers. Getting rid of metal rubbing contacts and minimize engine shakes will eliminate our woes.