Gutpile, I use the the overpowder wad to prevent the plastic shot cup from getting to hot,and possible melting. You can get a variety of wads from Circle Fly Wads. (www. circlefly.com) Ask to speak with Craig Smith.
The one I use is the 12 gauge cardboard wad. It can be used for over the powder, and overshot wad.

PS: I told you earlier that I used a 3" shot cup. It actually is the 3 1/2" shot cup!! If you need a good deal on these, leave a post on this thread!!