I used a slightly bent aluminium gutter hanger from Home Depot with felt liners inside to eliminate rattle and three bungie cords to secure my first set of kitty litter panniers with great success. Total cost under $5.
I just finished upgrading to what you are looking for by buying a full set of Ortlieb attachment hardware, but I warn you, it was about $60 plus $15 for shipping which is really outrageous, but does make the best panniers available at any price. We use the pails for seats when we are in camp and I have them set up so the tops are level with the rack so I have a giant flat space on top. I have a set of Ortlieb rear rollers too, but I now prefer these litter pails instead. Just was not ready to canibalize my rear rollers for parts so I bought new hardware (
http://www.ortliebusa.com/cartgenie/...st.asp?scat=21). I tried buying several brands of other pannier mounting hardware but found it either unavailable or more expensive.
You are on to the best solution for loaded touring!
Good luck