I make potholders quite often. I like to use an old cotton mattress pad which is thick and firm. If you use polyester, the heat will come right through. I make two rectangles, sew them together leaving an opening. Turn them right side out. Press flat. Cut a piece of pad to fit just inside the potholder. Insert it and flatten to corners. stitch opening closed. Stitch an "x" from corner to corner. This is the first machine sewn project I did as a 4H project almost 70 years ago. I do use insulbrite for my micro-mitts. They are an oval shape and have little finger pockets on one side at each end. I use insulbrite for these as it is thinner. Most of my microwave dishes are plastic or ironware and do not get extremely hot.