Before I cut the leather fender straps off, I threw the bags over and lined up one side to where I liked it and marked where the fender bolts lined up ( instead of lining up the other bag, I just measured inverse and marked it, so I know they are the same). The back side of the bags are a hard plastic. I drilled holes at the marks, got slightly longer fender bolts and a couple of 1 1/4" dia. washers for the inside of the bag to help distribute the weight a bit, and not break through the back of the bag. I've put some pretty heavy items, (couple of 2 liter Mt Dews
) in each, and various other stuff with no problems. They lay flat, unlike being thrown over the backrest, and they don't flap in the wind even at highway speed.
I don't know how long they will last, since the stress is only on two points instead of across the whole top, but the large washer do a good job, and since I only use them mostly to keep an extra helmet, jackets, gloves, and spare eyewear...I think it will last a while.
I'd post a couple of pics....but can't get to a camera or my bike right now