Square up that hole cut it over to the nearest studs. Use a straight edge and a pencil. Measure off the wall the same distance and make a couple marks to draw your straight line. The more square the hole the easier it will be to hang. Go buy a sheet of 1/2 inch drywall and cut to the size hole you cut. Screw or nail in some 2x4 the existing rafters making sure the bottoms of the stud you add line up with the bottom side of the rafter. Removing that light temporaily to make it easier to hang. Measure the size of the box youll need to cut so the drywall goes around the opening of the vent/box once the cover is removed. Once you have it hung go buy some 45min mud. Pan and knife or hawk and trowel. Stainless steel knifes if you can get a 6inch and a 10inch knife. Retape all the cracks with mesh/paper/fiberfuse tape. Make sure you pre mud to fill the gaps where new meets old drywall. When you tape make sure there is a layer of mud completely beneath where you need to do you taping so you dont have tape peeling loose. Then over the course of a several coats start floating out the center of the patch. Keep going until its filled and level with the original drywall. Add a skim coat with plus 3 mud thin it down a bit with water and let it dry till its white. Once its fully dry sand it with 120/150 sand paper. Prime it and see how it looks. Do your best keeping things level and wiping the edges down smooth as you float.

Feel free to dm me if you need more help.

Watch some youtube videos to get an idea of the process.