Here are some ways in which the makers may have produced an almost perfect round shape:
a) They might have used a round object to outline the shape, like a rounded stone or some sort of ball.
b) They might have added an inside structure (like a ring) underneath the hat to stabilize the curves around the object.
c) Bamboo is a strong material; however, with water, it is easy to bend it. In my opinion, I think they created an almost perfectly round dimension by hand without taking measuresments or using other tools to support the shape.