I wouldn't use the rayon one - you might want to ask the QUILTING bunch ....Carolyn

Be very careful of the batting you use in these. If you do not use 100% COTTON they can catch fire in your microwave. Don't ask me how I know so much about this....LOL. (the slight burn still shows) This is especially true if your microwave is in the higher wattages.
There is a batting made of 100% cotton especially for "tater bags". Some quilt stores carry it and it can be found online. Made by warm and natural.
As far as the 100% cotton batting you found, just make sure it is 100% cotton. In other words, read the small print and be careful.
I love my bag and use if for more than potatoes. Especially now that it doesn't catch fire!!