Here is an awesome sewing pattern for chemo hat. It's a modern take between beanie and turban. And in case you don't read Finnish it is emphasized that you do not sell hats made from the pattern. Is that too much to ask? From a free pattern? We'll we all know what the answer to that question is.
PDF pattern https://s3-eu-west-1.amazonaws.com/syopa-alueelliset/sites/274/2018/09/14144337/siskomyssy_kaava_a4.pdf
Pattern does not include seam allowances and they give 2 height options for the back.
Sewing instructions https://vimeo.com/219074989
Only for knits that stretch. Includes 2 sizes.