This might be to simplistic, but couldn't you take a length of material the dimensions of the sweat band cut to the longest length (which seems like the bottom edge, place it on your head (or who's ever head the hat fits). Then pinch the extra length, pin it with a straight pin. I would then cut off the excess sew it, check for fit. Once corrected now you have a piece you can use for a stencil. Sometimes a piece of string works the best, I have seen woodworkers do amazing things with just string as a measuring tool.