I'm in the process of finishing my basement and had built soffet around the I-Beams and the duct board that runs across the ceiling. The builder had cut a hole in the duct and attached a square grill directly to it for heat / ac in the basement. Unfortunately my dry-wall contractor covered it up when he did the soffets (my fault I should have extended it prior to him coming but I forgot about it). In any case I was able to locate the grill by poking a hole in the drywall but I'm not sure how to extend it down. I thought about cutting a hole in and sealing off the existing vent but I'm not sure how I'm going to run a new one. There's only about 5 inches between the duct board and the drywall and I haven't been able to find anything short enough to use. I realize one way or another I'm going to have to cut a new hole I'm just not sure what kind of fitting to use or if I use flex duct how to attach it to the main feed
The way I would do it is by using a 10"or 12"register pan.I would cut off the solid bottom of the pan leaving 6 1/2".Then you cut 1 " fingers.
That leaves you with a 5 1/2 drop from the duct.(if you are using 5/8" drywall then change the 1/2" part of my instructions to 5/8".)