Stick with all cotton fabric and batting and you won't have any problem. Be sure your potato is wet when you put it in the paper towel. I cooked potatoes in the microwave before I had a bag and allowed 3 minutes, stuck a fork in and if it wasn't done, cooked it one minute more. My microwave has a setting for a potato that works fine for me. Sweet potatoes takes a little longer to cook in the bag.
Didn't know there was a special batting for a potato bag, will have to look for that.
Carol J.