Just wondering if you have a trick for this. The canvas is wider at the bottom than it is at the top so as I roll it, the zipper sticks out more and more (as if I wasn't rolling it unevenly). When I get to the top, because it is wider it does not fit well with the other canvas'. Are there any tricks for this? The only thing I can figure is to remove the center piece. Thanks in advance for the tips...George