I think you would have had just as good results with a good set of foam blocks like these:
https://www.oldtowncanoe.com/catalog.php?fr_sw=0&igroup_id=146§ion_id=4 I've used foam blocks to mount the canoe to the roof of my Civic for years and drove 80mph on the highway without any movement ever. The crossbars on the Xterra are wide enough for the foam blocks to rest on.
I am not trying to put down the mighty-mounts. I've never used them or seen them in person, but since I don't trust the stock cross bars on the Xterra because plastic is used to mount them to the main bars, then the mighty-mounts were not an option for me. I don't know that stock crossbars are actually weak, but I prefer metal to plastic. So I am using u-bolts to attach two pieces of square tubing on the main bars to put my canoe on. This way I can tighten up as much as I want with my ratchet straps without worrying about breaking the crossbar mounts when I tie down the bow and stern.
I've put a piece of black hose on the u-bolt so it doesn't scratch the rack and I insert another piece of rubber between my homemade crossbars and the main bars for the same reason.
From looking at the pictures for the mighty-mounts and the gunwall brackets, I can see where you mean that support is lacking.