I agree with pillow ticking. Just be sure to get the kind that has the lines woven in. I Googled to find a place to purchase online.
Canvas does not guarantee not stretching. I had canvas leaders before the pillow ticking leaders, and mine got wonky on the diagonal at the edges -- probably from having the canvas loose while rolling smaller quilts. I am more careful about rolling now. I do prefer the pillow ticking to the canvas, though.
I didn't prewash the pillow ticking (sigh). Should have.
Next time I will use velcro to attach the leaders to the rails. Have seen some tips on how to do that. Use industrial strength velcro, for one. If you need to mark a straight line on the rails, a nifty way to do that is to line up two rails together, then run a marker along the line where they meet; marks two at a time.