One thing that I do is cut a piece of 4" PVC pipe the appropriate length and roll the target backers into those tubes. One end of the tube is glued shut, the other end is a slip-fit. It's a very convenient way to store and transport target backers from my shop to the range.
Oh, nice shop, by the way. I'm jonesing for a set-up like that and I've been planning it for several years. Hopefully in another year or so, I'll break ground on my shop. The land is clear and the plans are just about final. Now all I've got to do is get the money together.