I have 9 flex duct lines coming off a sheet metal trunk in my attic. I started sealing it last year, but quickly lost interest I was doing it the way I thought was right, pulling the duct tape off, putting HVAC mastic on the supply box cylinder, using a zip tie to hold that in place, and then using another zip tie to hold the outer sleeve in place. I'm just finishing up spraying Great Stuff into every nook and cranny I can get to and I'm not hip on spending any more time up there than I have to.
I have an idea motivated by laziness. However, I feel it's a good idea. What if I take the diffuser off from the ceiling and then just use some HVAC rated tape to seal the metal to the inside of the flex duct? It's 6 inches in diameter which doesn't give me a lot of room to maneuver, but if I can pull it off, would I really be losing out on a proper seal? I know I'll still need to seal the ductwork at the trunk from the outside, but will this work for the supplies?