I got a 4 x 8 insulated wall board( not sure exactly what it's called ) at Lowe's .. I think it's for putting under vinyl siding. There is different thicknesses, I got the 2 inch, then I covered it with headliner. It's the stuff that is used for the ceiling inside your car. I got this at Hancock Fabrics in Louisville,KY I'm sure other fabric stores have this. It works better than felt or fleece, I love it