I don't know if this will help but I have stretched at least one bonnet the same way we used to stretch our sailor hats ("Dixie cups") when I was in the Navy. Soak the hat in warm water then make fists with both hands inside the hat and twist the hat around your fists stretching and shaping it as you go. If it doesn't need too much stretching, that should work. If it needs more stretching then you can snip the back where the ties are, secure the ribbons (by sewing) to each side of where you have cut the hat, and then tie the ribbons together for the proper fit.