Once in a ski resort area, while on a business trip in early 1980s, I called the main desk of the hotel to ask for a heating pad for a sore neck and shoulder. The clerk referred me to the kitchen! There, the responder said he would "send me a pad" and then I learned about what you are calling "warmers" Since then I have used sturdy cotton like duck, corduroy, and channels (several inches wide), in any size anyone would want.