You can make a handle potholder (for cast iron pans) from a regular square or rectangular potholder. Take the potholder and fold it in half. Stitch across the bottom and up the open edge. I found whip stitching by hand easier than doing it by machine due to the thickness. You end up with a nice hanging loop too.
If you are talking about using these on cast iron, use old jeans, and sandwich worn out toweling or wash cloths in between. These pot holders can take everything but throwing them in the fire! If you are making them for gifts, use a cute fabric on the outside and denim on the bottom or inside layer.
Insul-Bright consists of hollow, polyester fibers needle punched through Mylar. The hollow fibers resist conduction, while the reflective mylar resists radiant energy. The energy, hot or cold, is reflected back to its source. Easy to sew, plus your finished project can go right into the washer and dryer.
The website sells Insul-Bright. Most fabric stores sell Insul-Bright or it may go under another name. It's for making potholders, hot mitts, tea cozies, etc.