I haven't made mitts, but I use cotton to pad my ironing board. I'm sorry I can't recall the name of it at the moment, but it is thin and comes on a roll, and may come pre-cut in packages also. You layer it to get your needed thickness. I think I got it at the Walmart sewing section the last time.

You can't use polyester batt, as it conducts the heat, that is why you use cotton batt.
Kathy G in MI