Wool is a very good insulator and if it's felted down completely it can survive trips through the washing machine. I have knitted potholders and felted them down; if you're not a knitter, you can still do this. Old wool sweaters can be washed in very hot water to make felt (just make sure the tag says hand-wash or dry-clean; wool can be treated to be safely machine washed). Thrift stores are a good place to look for wool sweaters; check the men's section for more yardage. ;-) There are lots and lots of resources online for felting knitted fabric, if you're interested in trying it.
ETA: Whatever you decide to use, do not use polyester. It will melt.