After reading a lot of this thread I checked each of my diffusers and was astonished at how much gap was between the duct and the diffuser housing. I spent yesterday using HVAC tape to seal between the housing and the duct.
Here is a pic of one housing and the gap. The white is the housing and that gap was big enough to put my whole hand in.
Here is a pic after sealing.
Had to cut pieces to overlap and made sure it was long enough to cover gap and have enough to seal properly. Turned the fan on afterwards and was very happy with the improved airflow. Next step is to block off the duct past the last diffuser. It goes for FEET past the last one on each end.
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