Nice, still seems a bit bouncy though. Thought about adding some bracing like extra rods towards the camera?
Another, simple fix would be to use a string, going from one corner of the square part, to the end rod, then to the other corner. Then tighten it enough so there is flex of the rod that keeps it from moving too much. Basically, a tripod.
I'd look into making a rig mounted to the bike as well, like 2 rods go to each passenger footpeg and one to the end of the bike.