Go to your local lumber yard and buy a piece of 1"x3". Cut pieces to fit the long edges and place them with 50% overlap above the existing drywall. Screw each piece in using drywall screws. Install your filler piece, I would suggest to place the screws about 6-8" apart. Use dry mix compound (Sheetrock/Durabond) for the first coat. https://www.homedepot.ca/product/saint-gobain-adfors-250-ft-fibafuse-drywall-tape/1000754764 Fibafuse is premium drywall tape and is better for butt joints. 2nd & 3rd coats can be premixed mud if you wish as it sands easier. Good luck.