I got up at 5 o'clock this morning and started researching chemo caps for an acquaintance at the library who will soon be losing her hair due to chemo therapy. It's hard to think of it as "therapy" since it is so close to poison, but its purpose is to save lives.
I found this one, which might work for summer here in Chico. The pattern says to use thin baby yarn which should be soft. This can be used as a liner for a baseball cap.