Thanks, everyone! I took off one of the canvas leaders and started measuring it. Doesn't make a lot of sense to me as I could measure only 1/4" distance off from center to sides, yet when rolled it was off way more. I took a closer look at another leader that is still on the machine and had exactly the same problem -- at least an inch off at each side. I think it is the result of a combination of issues -- dh stretching the edge too much when he duct-taped the leaders to the roller, the taping possibly off a bit towards the edges, additional stretching of the canvas (on the bias) towards each side, and the manipulations I have done sewing zippers onto the edges while they were still on the frame.
So, I am biting the bullet and making new leaders. Thanks, DogHouseMom, for the ticking idea! I went online to Beverly's and found woven ticking in a nice pink and cream. I will test it when it gets here to see if my poor man's snappers will work with its thickness. If it doesn't, I will sew fabric casings to the leaders *before* they are hung on the frame. I am keeping my fingers crossed that the stripes are actually even and on-grain.
DogHouseMom, what was your ingenious method for sewing on zippers? If the ticking is too thick for my snappers, I plan to sew zippers to the ticking so I can zip on fabric casings for the snappers. (Honestly, if I hadn't just written that sentence myself, I would have to read it several times to understand it!)