Hi BryanBowie and thanks for getting back to me.
OK: so I am using thules like you to use as a foundation for custom rack.
Like you, my "rack space" is divided in half. But unlike you, it is divided lengthwise. check out this picture:
Looking at your mounting, I take special note of that Thule through bolt. This won't work for me because the Hullivator surfboard rack (well, actually a modified kayak rack) base rests on top and sides of the thule cross member for the full width of the hullivator rack (or 1/2 the length of the thule bar). The protruding bolt head will interfere with this situation. see this photo:
I guess I could have the bolt head rest on the inside surface of the thule bar (requiring a hole in the top surface large enough to pass bolt head through).
Option I am chasing is to use those 100 series specific captive nut extrusion mentioned above, then purchase from amazon 2 sets (8 total pieces) of Thule 542 Artificial Raingutters (note that they can be used in the horizontal plane) and bolt these to the 100 series specific extrusion. I will use washers to initially handle the elevation mismatch (roof curve forward/aft). Once dialed in, I can have metal (aluminum) blocks made to replace the washers.
Bonus is that these extrusions will distribute the rack/contents load over the whole length of the forward/aft factory roof reinforcement....at least hopefully.
All this hassle to preserve my existing (and semi-well thought out & trail proven) compo cargo/surfboard (and paddle board) rack set up. Tried to find similar from all the rack vendors...the thule bar interface was the big stumbling block.
I'll keep an eye on this thread in case other design ideas appear...........good luck and excellent "thinking outside the box"!