Here's a link to Paragon Patterns. They have a number of patterns for caps, including what they call a "slag rag" to wear under a helmet.

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

Scroll down a bit to see the patterns.

You might find a pattern for a cap similar to this included in patterns for surgical scrubs.

CMC

Here is a link that might be useful: Paragon Patterns, caps

Looking for free pattern to make for cancer persond