I have two places under my main stair case where the air returns to the AC unit. Currently it is just grills covering it with no filters (There are two one on each side of the stair case). When I discussed adding filters to this area with the HVAC technician he seemed to think it would be fine and much easier given that location instead of the garage and having to open the main unit Everytime I want to change the filter. However I am somewhat confused. The size of the ducts on both sides opens at 11.5 x 11.5 inches. I don't see any air return filter grills that match that size. Do I need to just use a bigger return grill/ cut into the drywall in order to get the correct size or am I missing something. Thanks for any help.