Cut 1x4s to length, and then press them into the notches and tightly against the 1x2 corner cleats. Check to make sure that the 1x4s don't protrude beyond the studs. If they do, you'll have trouble fitting the drywall pieces back into the holes. If necessary, remove the 1x4s and cut the notches a little deeper or move back the cleat. Attach the 1x4 backing with 2-inch galvanized decking screws. Drive the screws into the notched studs and the 1x2 corner cleats.