DS is losing the hair on one side of his head, and I finally figured out it's from being in the sling! That's the side he rests on when we do a cradle hold or nurse. I'm using a 100% cotton sling now, and it's pretty soft. What might be a good fabric that's a little more slippery to keep from rubbing so much on his head? Should I get a new sling, or just put something under his head when he's lying down?