I once used a backpack called Studiopack manufactured by Designstar.  It seems to be a discontinued product.  It was big enough to carry an 18 x 24 canvas on stretchers.  It was nylon and slightly padded in the 18 x 24 pocket.  Among other features, it had 2 zippered pockets to carry tubes, a large center compartment to hold an artbox and straps to carry a easel.  Below is a pic of one I found online. Since it is soft/foldable, when traveling or at conventions, I used it to carry 18 x 24 to