I haven't tried to knit one, but I've sewed some. At it's most basic, it's just a long rectangle with rings attached to one end. I once made one using a 3 yard piece of fabric 45" wide just by tying the rings on with a necktie knot. 3 yards is too long, I discovered.
Anyway, a simple one would be to knit something at least 35" wide and ~2 yards long with rings attached to one end. The stretchiness of the knit would be hard to know in advance, but I would think you wouldn't want it tooooo stretchy.
A wider width makes for a deeper "pocket" which might be nice for nursing with baby in sling.
Hmmm maybe I'll try to knit one if I ever finish my sylvi.