I realize that this is an old thread, but it really helped me out, so I wanted to post some pics to help others. With just $5.84 at the local Ace hardware store, I was able to attach my Thule 835XT Hull-A-Port carriers to my Titan's overhead racks. I've been carting the 'yaks all over the place for the past couple of days.
I believe the factory bolts were M6's, but I removed them and used 4.5-inch long 1/4 bolts with washers and wingnuts. This worked like a charm...and I'm a very happy boy!

I tried the Horizontal Mighty Mounts with the Yakima carriers, but they didn't seem very stable. They are now on Ebay. I've always loved the stability of the Hull-A-Ports, so this was the best solution for me.
-Dan