I've finished my 1860's slatted sunbonnet. This isn't what most people think of when they hear the term "sunbonnet". However, the modern idea of a sunbonnet, think Laura Ingalls and Holly Hobby, was actually a Twentieth century garment. Those from the 1860's were much more utilitarian, serving a very definite purpose. These were frequently made of a lighter color cotton as it would be cooler in the sun, the wide brim provides a tremendous amount of shading from the sun, and the long "Bavolet" (the skirt of the bonnet) kept sun off of the neck as well as protected the shoulders and upper back of the dress from being faded by the sun.
This was quite an easy project and took less than a yard of fabric that I had in my stash. Is is two rectangles of fabric, sewn together with chanels stitched into the brim for cardboard slats. A tie in the back provides shaping that would fit around the hairstyles of the day, and utility ties keep it on. The slats are/were removeable for laundering. I had read that they feel a little like you're wearing a mailbox, and that's definitely the case. You have to turn your head to see what is around you, but definitely great for shade.