Using the option UI Scale, you can get a ton of extra space even with the default UI.

Also, there are six bars, so 72 buttons to put spells/macro's on.
Not sure if 72 is enough keybinds for every boxing composition.
Still, I believe you can hide the bars you don't want to see them.

Less things that can break in the UI is good.
Too bad the blizzard UI isn't as attractive as a lot of the addons out there.