From the
Fantastic Vent FAQ page....
Q. DO YOU RECOMMEND VENT COVERS?
A. Vent covers restrict the airflow of your Fan-Tastic Vent by about sixty percent. In addition, covering your vent requires more frequent cleaning due to dust and dirt accumulation. Although we do not recommend any cover over your Fan-Tastic Vent, it will not void your warranty.
To me, the biggest selling feature of the Fantastic Vent is it's ability to move volumes of air through the rig. By restricting the air flow by 60% defeats the purpose of even having a Fantastic Vent.