So I made one for me, and another for a friend having a birthday today. It has a decorative cover, which can be removed for washing. The inside bag is filled with rice that has been scented in some yummy smelling oil. I used a pomegranate scent and it smells super yummy. Just pop the rice bag into the microwave for 1-3 minutes, and enjoy the good smelling warmth for hours.
If you want to make one, here is the tutorial I followed.