If you wanted to add a strip of copper I think you'd have to buy it from a metal supply store. It's almost certainly going to be something you have to cut yourself or pay someone to cut for you.
It would be vastly more efficient then thermal padding I would think though.
But it's not a cheap and easy alternative.
If you have a metal yard in your area you could try scrap bins there, I get my aluminum and steel sheet metal like that, though I have not seen much copper.
Hope that helps.