The  front section of the air cleaner is inside the box.  This is the side where HEPA clean air exits the air cleaner. The air intake parts of the air cleaner are on the outside of the plastic box.  I cut the hole in the plastic box with a sharp serrated knife and metal "tin" snips.  I used silicon caulk to form an airtight seal between the  Holmes HAP-240 HEPA air cleaning unit and the plastic box.   I also used clear plastic shipping tape to support the junctions  between the air cleaner and the plastic box. I place the lid back on the plastic box when the unit is not in use.  I purchased the plastic box at K-Mart.  I think that it cost me around $10.00. The HEPA air cleaner was purchased at a local hardware store.  I have seen the exact HEPA air cleaner advertised at $49.00 several times recently.