We used to carry two 4.8m Penguins on the roof of a Rosa.
The following are pasted from two earlier posts
We carry two 4.8m Penguins on top of our Rosa. I figured that I would need to get up there to load and unload them so we had an alu rack made.
It is T shaped with the cross of the T at the back and covered with the checker-plate you mention. The members of the T are about 400mm wide. I can walk on all the T part. Then there are 2 cross rods of rectangular 20x50mm upon which are mounted 4 wooden cradles. The whole thing is bolted and glued into 6 plates welded to the roof, 2 large ones at the rear and 4 lighter ones at the extremities of the cross rods. It works well for us, with long kayaks and the need to be able to walk on the rack to load them.
You may not need something as substantial as that. If Sikaflex won't hold it, there are certainly industrial adhesives that will (as long as the paint holds!)
To load a kayak you:
lean it up against the back "scoop" (scooped aluminium tube across back),
Climb up the ladder onto the top,
Drag the kayak up over the "scoop" and into the fore and aft cradles,
Tie it down, fore and aft.
One can do it, but it is easier with two.