I picked up a Halo skull cap with a "tail" on it that covers my neck also. Happy with it:
https://www.amazon.com/Halo-Headband...%2C781&sr=8-11. Wicking material so it doesn't get hot. If your helmet fits tightly, the head band on it can feel a little tight. Luckily, one of my helmets fit a little looser.. I also have just a lightweight skull cap if I don't feel the need for the neck protection.