I would also just line them, but rather than muslin, I would use a polyester jacket lining type fabric. It is a little slippery which will make it easier for the pillows to be put in. I would cut it the same size as your pieced fabric, and treat it as one: put the wrong sides together and handle them as a single piece. You could add a few lines of quilting, but if you catch it in the 2 side seams and the bottom seam, then either serge or make a French seam, it won't be going anywhere. I would wash it first.