Not a pattern, but how you would create that, is to take your two rectangles of leather (or one large rectangle and omit the bottom stitch), stitch the bottom and two sides and attached your zipper to the two top edges. Create the square bottom of the bag by cutting rectangles and then stitching the two sides together to create the box. On the upper edge, don't do this. Flip the bag to the outside and you will have your tie down section because that triangle would have been your rectangle if you wanted to box it.. You see this done a lot in dopp kits but I've never seen on in a duffle. Looks great!
