More info re Ann's suggestion; see the screen shot.
In my example, I set the "Zone" to upper right, Right aligned, and top aligned. And I have made the box an exact left to right size, and large from top to bottom. As you add lines, they will stay at the top and move down. Or if you set Center Text Vertically, a one line caption will be in the middle, and as lines are added, they will move to stay centered.
I assume you are burning in. A sidecar file is unlikely to work.