I am doing some renovations in my 1950 ranch style home and I have gutted the living room of all the drywall/plaster. I have two large 4"x10"air returns on the outside wall under a large 62"x68" window. In the basement they have used the joist bays as the return space. I would like to remove these from the wall and cut returns into the floor. My question is can I just use wood blocking to seal up the joist bay end as to cutoff air flow from the wall. I would then also seal up the sill with wood blocking as well so i can insulate this area. Does this sound okay to everyone? I know they have used sheet metal to seal the other side of the joist bay but my sheet metal working skills are not the best.