If you are wanting 'heavy duty' handles, rather than just stitching them into the seam at the top of the bag, have them meet at the bottom of the bag, stitch along the edges of the straps wherever you can on the body of the bag and then do an 'X' at the upper edge where the handle and the body of the bag meet.
This way if you are carying something heavy (like books or a laptop) the weight is more evenly distributed.