Drywall hole slightly too large for mounting bracket to cover
You can purchase a piece of scrap drywall from your local hardware store - they might even give it to you free for that matter. 


Make a rectangular piece the same size as your box. 
Drill the hole for your wire.
Install rectangular dry wall piece over the box, patch nicely. 
Sand and paint if need be.


When I do this I usually run a screw (or three) part way into my drywall scrap piece - so I can hold it in position in either manually or in the case where I have 3 screws with string / wire to keep it where I want it - when patching a wall.