^^^ I don't see a photo or drawing of the alcove in question, so I'm half guessing. But putting traps across an opening generally works well.
--Ethan
Ethan, there is a photo of said opening/alcove on the first page of this thread. The speaker covers much of the view, but if you follow the wall corner down until it disappears behind the speaker, then that should give a pretty good idea of what the alcove looks like. I also have a Mondo Trap on the wall beside the speaker (which again, you can't see because the speaker blocks the view of it).
Better yet, here are some additional photos of the offending alcove: