I'm a homeowner. I notice if any of my air conditioning vents (I guess that is the name of those metal covers that stick on the wall/ceiling over the opening to the a/c ducts, that have metal vents on them that you can open or close to let more air out or not) are not flush to the wall/ceiling/floor then the air will escape under the edges of the register and can form mold (cool air and some condensation). Not wanting that, I thought maybe I can go (somewhere) and get some sort of gasket that will provide a better seal between the wall and the register.

Is there such a thing as a register gasket and are they good ideas to use? Can I get them at my big orange box? If they aren't a good idea (or if they don't exist), what is a better way of making a tigher seal between the wall/ceiling and the register? Thanks.

I also have one duct that has a piece of sheetmetal that sticks to far out of the ceiling and I was thinking of cutting it down a bit. Is that a good or bad idea?