I definitely watched the video (helped a lot - thanks) and I see where you mentioned you added tape on the exhaust outlet from the heater as it was loose. That might be my issue as mine too has some play. Mine definitely wants to sway a bit. I suspect this concentric kit was designed for b-vent which would be snug and the gaskets came latter to factor for the space of 4" stainless given the newer codes. The gaskets help a bit, my kit from greenhouse did not come with the blue gaskets and I may have to call about that. Though I doubt that would be enough. I may have to fashion a support.
Thanks for the quick reply.