with record heat for Seattle area (41C yesterday) it became painfully obvious that the HVAC system vents are dumb and would need tweaks. Cooling an inner bathroom is pointless, when an outer perimeter bedroom is toasty. The solution is to restrict the airflow with in the bathroom, which (I guess per building code) doesn't have any controls to restrict airflow.
Here is a model to do it in a reversible way.
It friction-fits within the default 10 inch register mesh.

Print with no top and bottom layers, and adjust the grid infill % to arrive at optimum airflow (see screenshots).

Prusa

i3 MK3

No

No

200

38

Print with no top and bottom layers, and adjust the grid infill % to arrive at optimum airflow (see screenshots).