Our old oven mitts have become so greasy over the years that even the washing machine cannot deal with them any more. So I have decided we need new ones. Although the store-bought ones are quite functional with their heat-reflective silver fabric, they can only be washed at 30°C which is not enough to get the grease out.
I have used some old canvas scraps, a cotton lining and an old cotton towel as the insulating layer in-between. I have admittedly not done a scientific comparison with the store-bought version, but I have used my mitts a few times now and never burned my fingers.