I've never done a summer quilt using muslin as a base. However, the stretchiness of the strings probably require some sort of stablizer. You might try heavily starching the strings' fabrics so that you don't need the muslin. This would probably solve the bulk issue and make the qulit lighter, and the quilting would hold the layers together. The summer quilts I've made just have no stabilizer in the blocks and a fabric backing. I was surprised at how warm it was.