just restrict the flow of air. you can block the opening (foil tape, an obstruction..) or crimp the ducting. this WILL work, but it will also put undue wear on your fan and is a waste of energy. I prefer to waste my energy on lumens


if you don't have one already, a can filter will also slow down the air flow. get a smaller can filter, cheaper and more restrictive.. and you kill some smell.

you will need one later!