What's the right way to expand the box around the subtitles?

[xereeto]

Been messing about with the code of OMXPlayerSubtitles.cpp and SubtitleRenderer.cpp, but I can't quite figure out how to do this. Trying to make the black box bigger so the text doesn't go outside of it. I can add horizontal padding OK, but trying to add vertical padding results in gaps between each line of subtitles for some reason. Is there something really obvious that I'm missing?