I am working on exactly that same set up right now.
I found the link to the PackRat Pelican system very interesting. I bought a rack from PackRat but had Mark modify it for the Pelican box. He has since taken this further although I would prefer my rack over the current one. Looks to me that he is bolting the boxes on which I don't like.
I want something that is easily removed. I made an "h" shaped bracket from aluminum and bolted them to the boxes. I am still trying to resolve the upper mount with some type of a latching/lock system. I just returned from a week long trip with my boxes mounted. The "h" brackets worked perfectly and I just used a heavy wire tie through the Pelican case handle and to the KLR rack to hole the bag on at top. I could still access the cases when on the bike and at the end of the day I snipped the tie and took them off.
Here's a picture of the PackRat custom rack. I'll take a picture of my Pelican box later today.