I kept going on and on about cotton but this footwarmer is made with white linen which is also microwave-safe as long as it's 100% linen.  I lined this one with the wrap-n-zap batting so it's soft.  And there's my cup of water to remind you how important that is!

The "warm tootsies" machine embroidery design is a freebie in my creative fabrica shop if you want to download it from there.  https://www.creativefabrica.com/product/warm-tootsies/