It would be done the same as a dust cover for a toaster or mixing bowl. You might be able to find a pattern for one of those. If not, you can just make a cube missing the bottom. Create your pattern by taking a piece of newspaper and folding it over your featherweight. You can tape sides on it, then cut it apart, add seam allowances and sew together. I would make it either with pre-quilted fabric, or pre-quilt your own in the right size. I would cover the seams with bias tape (or even straight edge binding). It can go either inside the dust cover or outside.