Another DIY noob here: I would make a cardboard template the size of the opening, then use that to pattern to a strip of drywall. Cut, chip and break the drywall to get that oval opening around the ducting. Concentrate on fitting the opening closest to you (as you stand there on your ladder); an insulation strip of foam can fill the gap. Since the ducting touches drywall on the right in this picture, just cut two more small triangles to fit the unseen corners closer to the wall. I would adhere those to the wall/drywall, then tape them in place until they dry since it looks like there’s no wood to screw them to. Mud the gaps, paint the repair and then the ducting to match the wall or ceiling. Then stuff the insulation strip around the curve of the ducting. Otherwise box it in like previously mentioned.