I have used cotton and fleece and love them both ways.

I understand that if you're heating it in the microwave, everything should be cotton, including the thread. Otherwise it could cause a fire in your microwave.

If you want make the pad in cotton, then a covering could be anything that you want and you would be able to wash the covering without damaging the rice or any herbs you might put in with the rice.

Always use 100% cotton as tilde stated. I like making a bag for the rice using muslin then a washable cover of flannel--sort of like a pillow.

I have used a terry wash cloth, folded it in half and sewed up the edges. To make it easy to launder, use a separate bag inside and velcro closure on the wash cloth. This worked very well when the veterinarian said to apply heat to my dog's ailing front leg and I did not want drips from wet compresses. Judy

P.S. If your microwave is high wattage, your cotton may still burn - ask me how I know.

So you may need to heat at 70 or 80 percent power.

Found a couple of cute tutorials.
Will 20 minute long grain rice work? I don't think that's considered instant is it?

Hi sewmom, I make my heat packs out of cotton corduroy material and fill them with wheat and lavender, I have also made washable pillowcases for them, so when it gets grubby you can put the pillowcase in the wash. Hope you find this helpful. Thankyou for sharing. Kind Regards Lorraine McNeill of South Australia

I have heard that you should use only cotton, and even cotton has a small level of fire danger. I would not use anything synthetic.

my microwave did not like cotton or felt, but fleece was ok. I don't know what you have decided to use

I have always used cotton fabric and barley. Never a problem and they have lasted a very long time.

You can use any cotton fabric but I prefer flannel. Then I make a removable cover out of flannel or fleece. If you are concerned about putting fleece in the microwave you can put the cover on after it is out of the microwave but I have never had a problem with it. And I use corn insteag of rice becausr it holds the heat longer. You can purchase large bags of corn in sporting goods depts.