Agree with 80/20 Hobbs batting. Note that it is supposed to be quilted no more than 4" apart, so keep that in mind while quilting so that it won't come apart with repeated washings.
Another option is to use a good quality flannel with no batting. Wash it and dry it twice before using so that it doesn't shrink up. It is important to use a good quality flannel, which can be pricey, if you decide to do flannel.