You can bring the back to the front for a self binding finish. Since its denim, I would leave it an inch bigger all around, fold it over the front and stitch it down leaving the edge raw. I would then snip to the stitching line every half inch for a fringe raw edge. You could turn the edge under and top stitch but I don't think you will not be able to sew through all the layers especially in the corners.
If you want a clean finished edge on the self binding, it would be easier to leave the cotton side bigger and fold it over the denim. A self binding is done much like wrapping a present and you fold the corners in first.