Start with a 18 x 24 inch piece of paper . Fold a set of side extentions from 2 x 12 in strips of paper.
Fold them into a "u" shaped position. They will look like the handled on brown paper grocery bags. Glue them to the top opening of the case.
This holds flat artwork with out folding it if the art papers are 12 x 18 inch or less.