I bought an aquarium filter, I don't know what it is for, It less than $1 for one big green/white filter. I modded it in front of the fan,for the side fan behind it. It works! And its cheap. I attached it with Tape

Pc also more quiter now. I did some test and no restriction to airflow. I just started placing it, and already I see mosquitoes stuck on it. So I recommend fish filter (I don't know what I bought) to anyone. Only make sure you have 1/2" space, its a bit thick.