I just made the same one.(Mine)
All I did was use a 2.25"X6" strip of the same wool. Fold the length so that the ends meet in the middle. Sew along the top and bottom edges, then sew it onto the bonnet. As you can see in the last pic in the link above, it does not droop at all.

Yours looks really good, btw. I may have to re-sew the top of mine so that it is not so floppy.

T