From Waves & Wild:
The Fell and Dale is traditional style Flat Cap. Medium weight woven fabrics such as tweed, corduroy or denim are perfect for the Fell and Dale Flat Cap. The Fell and Dale is ideal for intermediate sewists. There is also an option for adding elastic to the back which makes the fit/sizing more flexible.
The instructions give detailed directions for suitable fabric choices (and don’t let the idea of a peak/brim put you off, there are several alternatives including re-purposing a milk bottle!)
Sizes: 11 sizes from Newborn to XL adult.