Here's a few ideas:

See the link at the bottom of this reply for an online pattern.

Some other places to check - You can buy a pattern here:

http://www.paragonpatterns.com/Accessories/accessories.html

If you Google "chemo cap" you may find an online pattern for a du-rag type cap. There are a number of free patterns online for chemo patient head coverings, but not all of them resemble du-rags. The following "chomo cap" design is close to a du-rag, but you'll still need to purchase a pattern:

https://www.nancysnotions.com/NNVia/CustomPages/gbmarcharticle.htm

HTH,
CMC

Here is a link that might be useful: Online Free pattern

I had 2 sites for this pattern. One was exactly what is listed above. the other is non-functioning. I will check to see if I can locate it and send it later.
HAPPY HOLIDAYS

Here is a link that might be useful: http://www.ridemyown.com/articles/other/pattern_doo-rag.shtml