Not everyone has fiberglass resin and mat at home. I'm willing to bet 95% of folks some JB though. If you put a thin piece of sheet metal on the inside radius of that vent to act as backer, then fill with JB, it would work fine.
If it were me and I was going to use what I have on hand to get it done I would wrap the outside of the tube with tape and fill it from the inside, then once set pull the tape and see if it needed any final touch up where the tube slides on.
It's just an air tube.
Were driving Rusty nuts with the discussin it for days. :D