The 27" is a normal width for car seat cushions. A person needs the wider batting for padding the top materials. Cotton or polyester batting is basically a choice of authenticity but the polyester is fine if a person wants to use it. It's a bit more expensive. When using cotton, it's best to get the least amount of linters as possible. Cotton pulls apart and can be adjusted for layers or splices a slightly better that polyester but it's kind of like an apple and an orange. Both tend to slip under hard use and that has always been somewhat problematic. Some folks find ways to tie it to help keep it from moving under the cover materials but that is where pleating helped out a lot. Some model A seats are pleated and some are not. Those that are not generally had the cover material joined in two pieces across the side to side width of the car seat to help keep the batting from moving around. This link illustrates what I'm referring to as to non pleated covers. It's not exactly like they originally were but a person can see how the cover was put together.
https://automotiveamerican.com/2020/...refurbishment/
Look for the arrows on each photo string so you can see all the photos.