A layer of flannel that has been washed and dried a couple of times would work well. Quilter's Dream Lite would also be a good choice. Warm n Natural has a stiff drape, so I wouldn't use that.
I prefer cotton battings because they wear very well over time -- provided they are quilted closely enough. Most of the antique quilts we treasure so much have cotton batting. An all-cotton flannel will give you the thinnest batting possible; just be sure to prewash and dry. Flannel can shrink unbelievably; two pre-washings and dryings is not too much.