Sorry, I can't answer your question. I haven't made one yet but, I have had lots of flannel for quite a few years and have toyed with the idea of making on or more of them. I'm afraid of having all that lint going through my dryer. I wouldn't want to worry about a dryer fire.
I'm sure someone will have an answer for you as I have seen lots of them on here. I think I would use just a couple of layers of flannel with the top. It probably depends on how warm you want it to be.