Two models favor a downward direction: DB-4D and DB-3D.
The DB-480 is the most versatile model, for both upward and downward exhaust direction.
For Upward Venting Situations: Remember, for dryers on a pedestal or a stacked dryer with a preferred DOWNWARD direction of airflow, an "UP Box" (DB-425, DB-350, RB-400, RB-500 can and should be installed upside down.
For Downward Venting Situations: Use a "DOWN box" when the dryer is a standard free-standing dryer and the venting direction is down. Examples of Downboxes are DB-4D, and DB-3D.
Also consider the universal Dryerbox DB-480; this unit was specifically designed to work in an upward or downward flow direction and it is also tall enough to capture exhaust heights from a stand alone dryer or a pedestal dryer.