I use 3/4" pipe insulation, its the kind that is preformed for copper and steel pipe. I just cut a piece of it to fit under the shell holder and trim off about 1/2" off each side where it butts together. Usually its got a sticky end where they butt together. You can also tapper the 2 ends that are cut to lenght. I then use some electrical tape and tape the 2 ends to the stock. Slip the shell holder over the insulation keeping the insulation completly under the holder. If that isn't enough just use 2 pieces. It also feels good on the chin also. Any hardware or lumber yard will have it and the cost is cheap. Zeak