How to close gap between supply duct and drywall/diffuser

Hello,

Most of our registers are floor level, but several years ago our HVAC company added supplies to the part of the house that is on a slab. Those supplies had to go up into the attic crawl space and down though openings in the ceiling.
Over the weekend, I removed the diffusers (register covers) to paint the ceiling and was amazed to see how big the gap is between the duct-work and opening in the ceiling. See the picture below:


We must be pouring conditioned air into the attic crawlspace. That part of the house does not heat/cool sufficiently as it is. We can't afford to be throwing away conditioned air.

I was tempted to use foil tape, but I think that would just detach over time. What would you suggest for a good material (or materials) to close that gap?

Thanks in advance.

 

I don't see anything sticking to the exposed gypsum edge of the drywall so I'm thinking some kind of angled metal like corner bead screwed into the inside of the vent and the ceiling face of the drywall. You'd have mudding and painting to do in all likelihood, though.

 

Even screwing or pop riveting sheet metal to extend the duct should help a lot, maybe foil tape the seam.

 

You can get a duct collar ,cut it to the right width, slip it around the outside of that duct, pop rivet or screw them together, then tape the seam between them with foil tape.