Filter holder for Cat litter box

I built a custom litter box for my cats (a second one) and this time I incorporated a carbon filter / air vent. To hold the carbon filter sheet, I designed this custom holder, composed in two parts. 

The first part is the structure holder, is mounted on the litter box wall, around the “vent” slots, with 8 4mm screws (which is overkill). You could put 4 screws and it would be just fine.

The second part is the filter holder, designed to have a 150x165mm carbon filter sheet like this.

The two parts snap fit together with 4 indentations I made around the perimeter of the holder.