Here is how I modified my throw-over Ortliebs to attach to Givi PL532 racks.
I started with user Rbertalotto's idea (thank you BTW) involving a notched aluminum strip at the bottom to accept the bottom pegs on the Givi racks.
At the top center of the bag, I installed a 3/4" cam lock from Ace hardware (P/N 5302583, $6.50), with the keyed end on the inside of the bag. The latch part moves 90-degrees, from vertical to horizontal. Then I installed another strip of aluminum between the bottom aluminum rail and the cam lock to keep the lock on the same plane as the back of the bag.
Hard to describe, so here are the pics.
This is in the "unlocked" position. The aluminum is 1/8" by 1.5".
View inside with keys. My allen bolts weren't quite long enough to engage the nylon on the NyLock nuts. Looks like another trip to the hardware store is in order:
The barrel of the lock is below the upper rail of the luggage rack.
Another view. I added a few turns of electrical tape to the lock barrel to take up a small gap that was between the barrel and the top rail of the rack:
I cut off the large velcro straps, and left the smaller 1" straps that secure the bags fore and aft. This gives a little more support & strain relief to the cam lock. My reasoning is that I have a Chinese-made cam lock holding German-made bags to an Italian-made luggage rack, which is bolted to a Japanese motorcycle. Which part is most likely to fail? 8)
The cam locks are keyed alike, so one key will unlock both bags.
With 4' of aluminum, the leftover was about 8".
Overall costs were about $35, including all stainless metric hardware, 3/4" hole-saw, 4' of 1/8" x 1.5" aluminum, and two cam locks.