Billr- thanks for the reply. That's about what I had in mind to do. I was thinking if installing thick styrofoam-like insulation especially on the underside of the flooring. Would glue and thin metal strips work out?
The ducting is so agap that I can reach up and put my hand inside. So, I was thinking if removing the duct and closing the chasm with double stick foam tape. I would use sheet metal screws or pop rivets to close the chasm. Any comments any one?