Help Needed

A friend and I are doing a quilt together. She will be embroidering verses on blocks, and I will be doing the piece work. This is the pattern that we love.
My dilemma is what material to use for the 6 12" blocks in the center. She needs something stable enough to embroider on. What do you all reccommend?

This is the same quilt done many years ago. I foolishly used muslem in the center 12" inch blocks. We know that we need something more stable.