FoilMastic 367-17

[Theshandman]

For anyone looking to seal up the foot boxes prior to installing heat shield material and/or carpet, I have found 367-17 to be quite effective at sealing panel junctions. I had a spring tuneup on my home HVAC system recently and the tech repaired an old duct joint with this stuff. I was impressed with its stickiness. It's malleable and press-seals around rivets and of course seams. Not cheap if you go with Nashua brand (about $80 for 100'; couldn't find less than 100' rolls) but I found Polyken brand on eBay for half that cost and it seems just as good.
166015

[GTBradley]

I’m guessing it doesn’t tear as easily as the other stuff? I like the foil duct tape but it does tear easily.

[Theshandman]

I’m guessing it doesn’t tear as easily as the other stuff?
Right. It doesn't tear very easily, and the mastic won't let go of the foil if you do try to tear it; it just stretches. A sharp pair of scissors does the trick nicely.