Long wall or short wall?
If it's the short wall, then dipoles come to mind. Dipoles can be placed right smack up against the sidewalls because they naturally have a nulls to their sides, but they do like to have a fair amount of distance behind them, at least 3 feet and preferably 5 feet or even a bit more. This distance from the wall behind them imposes a time delay on the arrival of the backwave energy, and ime this is highly desirable.
Duke