I took my Grandmother's old (might have been made in the early 1900s)
sun bonnet apart and made my daughter and sister copies. They wore
them to a Civil War reenactment and they looked so authentic that several
women offered them up to about 40 dollars for them. You might borrow
a real sun bonnet from an older friend, or find a pattern for them.
Mine used strips of cardboard originally, but I used strips of a bleach bottle
instead. They kept the visor stiff and straight out. I also used a much washed
and bleached old rose pattern cotton dress for one, and it looked lovely.