Patching Drywall – How to Repair Drywall with Electric Cable

Since you want to have the drywall piece be 5" high I think you will want to have backer behind it for its full height. Consider making it 5.5" high and then using a piece of 2x6 lumber (that comes 5.5 inches wide) as the backer piece. Cut a couple of pieces to straddle across the bottom of this opening at the back and middle to hold up the shelf the full length. To install the front backer piece pre-drill clearance holes through the 2x6 that let your screws slide in place. Then use screws about 3" long to secure through the existing drywall at the corners. This will end up surprisingly strong! See picture below.

The trick to get the 2x6 to screw in place would be to start two screws from each end on each side (eight screws total) and tighten them down evenly and gradually. Make sure to stagger the pre-drilled holes from side to side of the 2x6 on each end so that the screws do not collide with each other as they enter the existing corner studs.

Hopefully there was a notch drilled in the plate, so that when a wire staple is set it will pull the wire in behind the face of the drywall. It should have been cut out large enough to allow a metal plate to cover the wire to protect it, although it is unlikely there will be any nails or screws going into the corner right there, but you never know....

Anyway, after the wire is held in place with a staple, pack the slot with drywall mud if you prefer, but it will take a long time to dry, unless you use a fast setting type and scrape that flush with the wall, do not allow that to build up or sag. Add another layer of mud for a setting bed for the tape, whether it be paper or mesh. Set the tape over the whole slot, running it into the corner and out onto the wall and ceiling. It may be a little tricky, but you can do it. After that has set add you finish coats, let dry and sand smooth.