I am trying to move the return air location for my house to make room for stairs (converting a duplex). I basically want to move it across the room into what will be the pantry, but trying to minimize the space it will take up. I have a 3.5 ton unit for about 1050 sqft and before the filter they were using was a 14x24. I have the old air return duct under the house but it's very large and I don't really want to run that up through the floor in the pantry. I'm thinking i'll need a larger filter (20x20 or 24x24 or something in that range) but not quite sure how to build the return box or the best way to run ducting to it to minimize the space it takes up. Where I'm trying to put it is roughly the center of the house. Any suggestions on placement, size, box construction? I'll try to include some photos later.