For the seat bottom, I wanted an extra layer of protection for the upholstery, figuring that the seat bottom is the one that gets the most abuse. Here we have a 25"x22" piece of batting sandwiched between two pieces of muslin, sewed together at the edges. This is a bit thicker than the original batting, but the upholstery will go over it, and doesn't seem to stress the upholstery thread too much. Unfortunately, this won't work for the seat back, as the upholstery is a much tighter fit there (barely goes over the batting, and I had to re-sew a couple of the seams of the seat back upholstery).