Another thought, some folks make straight stacks by simply cutting the large spark arrester away, leaving just the inner pipe - though this does look (to my eyes at least) a little odd, since the stack then tapers gently from the smokebox end but then abruptly goes vertical, because that's the shape of the inner pipe "core" inside the large spark arrester. I have a couple of Stainz conversions that have had their stacks done this way, and I'm not particularly happy with them, though their builders obviously thought it looked OK!
Jon.