IF you must use that ac vent to blow air into the grow area... i would recommend that you get some card board, about two pieces just larger than the ac vent. Cut out a hole in the card board slightly smaller than the flex duct that your using and then place the duct into the cardboard. then tape up the card board over that ac vent. you can also remove that ac vent frame, and screw in the card board over that using some scrap plastic pieces to use as washers or so. The point is to make a better seal but still thats far from an appropriate set up. the way you have it set up, you dont have as much pressure blowing cold air through your duct as you could have.