Read over a recent similar thread about seat buns. That owner decided to seek a professional at his last post.
Replacement buns will need to be shaved to size. A piece of muslin cloth is glued over the shaved area just to smooth it. Foam doesn't cut pretty,( how rough varies with the tool used ). I used electric carving knife. Then slots cut for listing rods. Hog rings through holes are daisy chained ( 3 in row if my memory serves ) to pull/hold the listings down in place.
Having originals to take apart / use as guide makes it all sooo much simpler.