The outlet vent must be at the top/bottom part of the enclosure but not necessarily at the front (Can be at the back, sides, and top, in case of a drop wall), as long as the measurements below are maintained.

For Flare See Through fireplace, the open vent must exist in both sides of the glass

From the manual :

Top vent system – Hot Airflow outlet from the wall enclosure MUST remain open. For Flare See Through fireplace,
open vent must exist in both sides of the glass. The outlet must be at the top part of the enclosure but not necessarily
at the front, as long as the measurements below are maintained.

For Fireplace size above 60″ / 150 cm – Vent area above 140 Sq inch
For Fireplace size below 60″ / 150 cm – Vent area above 100 Sq inch

Lower vent opening –  Optional for normal installation. A Must for double glass installation or blower kit option.

For Fireplace size above 60″ / 150 cm – Vent area above 70 Sq inch
For Fireplace size below 60″ / 150 cm – Vent area above 50 Sq Inch