We have a commercial one in a double layer of dark brown cotton that lives in a drawer. It works pretty well I think, although you do occasionally have to cut off a shoot or two. When we have too many for the cotton bag, we tend to just put them in next to it, and they seem to do ok, too. I think it's the dark that matters most. if you grow potatoes, it's the bits that stick up out of the ground that go green, hence the need to keep earthing them up.