Your potholders really looked nice.
For me, the trick is to find something that protects me from the heat - and will still be flexible enough to bend fairly easily.
I've found felted wool sweaters to be an excellent filler, terry towels, table protector stuff (SHRINK that stuff first - it shrunk about 30% when I washed it)- cotton battings.
I also do not use insul-trite or the ironing board stuff. I also avoid anything polyester for the filling.