I had a similar problem with my weedeater years ago. I couldn't get my walkman to work without serious distortion when it was running. I found that aluminium works best for EMI shielding. I built a little box that I clipped to my belt, that the walkman could sit in. It worked like a dream. If you can find the precise source of the EMI, you could either build a shield around it or at least between it and the amp. You have make sure though, that the shield is grounded to work properly. You can even paint it to match whatever you want. DON"T use the mylar bags from computer parts. They don't protect well from EMI. Just static. And they are flammable too!!!! As for your spark plug boot, you would be better served with a suppression cable as well. I think the real thing to figure out is, is your problem caused by EMI or RFI. Both can play havoc with electronics but each has different solutions.