We are doing a kitchen remodel, and the new configuration will have a cabinet in front of our heat register. So, our installer made a new floor register for us using some flex duct. The original register needs to be sealed off, so the installer put a piece of drywall over the opening. However it seems to be leaking a bit (into the wall and basement)and I am wondering if there is a better way to seal off this duct opening (the duct opening is in the wall, at the base of the wall). Any ideas?

Thanks in advance.