Personally, I'd get a swatch from Rocky and use fabric glue to glue the extra fabric on the back. Be sure to arrange the tear so it's as invisible as possible. If you feel it neccesary put on a patch of denim first, then cover it with tartan so it doesn't show if the apron blows up a bit. I would use regular denim fabric rather than an iron on patch, fabric glue will work better. A little care and it'll be hard to find the tear at all.