I made mine out of flannel and used cotton thread. You don't want to use anything but cotton to make these.
Depending on how much rice you use, will determine how long they will stay hot.
You may want to consider making a sleeve to slip them into, so that if there is a spill you are able to wash it.
They won't keep a dish hot like a chafing dish will, but they do help to keep it warm. I have used them packed around a casserole dish while traveling to a dinner :D:D:D