I don't know that much about it, but I would think that 1. The cube would be better (since you're not overlapping the rear wave...though I don't know how much that really matters) and 2. That you'd be better off with a different cover...something with some sort of fabric wrapped around a frame and installed over the gap. I'm not sure if the metal grate would affect the sound...but it might start making some of its own (rattling, etc.) when you turn up the juice.
Hopefully people who have done IBs before can chime in, though.