Yeah, I asked the seller of the one I posted and they said the 18" won't fit the 21" seat I have. I'll have to figure something else out.Tallest seat I've seen a cover available for is the 18" seat back. I just ordered the one below for my 2520; I measured my seat and tried to put the "medium" cover on, I have one of those on my 318's seat. It would fit, but stretched the back too tight for it to last. I knew then that the "large" would fit my 18" seat. I'll post when it gets here and I see how loose or snug it is, maybe itwould work for you. The literature says "up to" 18", though.