Are you planning on quilting the top? If so, I would most likely use a light weight muslin (won't be seen) under the patchwork. I'm assuming that you don't want to use a batting. If you want to add puffiness, then the most light weight batting you can find, then attach a sheet to make the slip cover.
Hey, I remembered something, I went to Bed, Bath & Beyond and they have these neat little clips that hold your comforter in place, mine always moves around, works great.