So I wanted the same thing and looked into this option. Once I looked up the tube, the design efforts and all the hardware, I realized the guy at Caribou Luggage isn't asking very much for his part on his design efforts.
I went ahead and just decided to go with his system so I'll try to detail for you what they did and what's great about it. This system is based on Hepco Becker racks.
First off, the racks mount up from the 2 pairs of under bolts on the back (one of which supports the muffler hanger). Those 4 bolts do most of the heavy lifting and the bottoms are mounted to the tops of the pillion peg extensions. There is a transverse bar across the back to lock the 2 racks together. If I was building one, I'd make this bar either a half circle that stuck out from the back or went up, so as to give the rear tire room for full articulation. I found this out the hard way this past weekend, fully loaded out in the desert. I came off a big jump and heard a big boom. later saw that the back tire had risen up and hit that bar and caused it to bend up into a V. I was able to straighten it out, but I'd like to modify it to prevent this from happening again.
The attachment system they use is bomber and should never come off unexpectedly except in a hard crash, as they explain. I carry a coupe of the spare lock pins should this ever happen.
The cases are fully lockable and secure. I have no fears of someone being able to get any of these panniers off my bike without serious tool intervention.
I also mounted "tool tubes" vertically on the back sides of my side cases where I keep 2 MSR fuel bottles, giving me some extra fuel range.
Good luck in designing your own. I have to give them credit where due, the guys at Caribou have taken this challenge and succeeded well on it. I'm very happy with my system.