My only idea was to buy one of those helmets that kids with sudden seizures sometimes wear, but I knew they had to be expensive and not easily found. I was thinking of fashioning my own from foam rubber when the idea of using a t-shirt shirt sleeve came to me.
I took an XL size t-shirt sleeve and cut it off. Now we have a pretty "hat" that keeps us from scratching. If she would get it in her mind to take it off, I would just use a clip at the back of her head to keep it on, but luckily she thinks it is pretty.
UPDATE: The XL sleeve stretched too much and would slip off too easily. I moved down to a Medium size sleeve and it is quite snug and less prone to stretching enough to fall off.