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).

Print Settings

Printer Brand:

Prusa

Printer:

Rafts:

No

Supports:

Resolution:

200

Infill:

Filament:
HobbyKing PLA blue

Notes:

Category: Household