Given the shape of the room, have you considered doing a little theater-in-the-round, and set up a stage in the center of the room. Otherwise, those in the back are going to feel a little out of touch with the singers.

For staging, you can always go buy a bunch of concrete blocks and 3/4" plywood from your local Big Box Building Supply Store (Here it would be Home Depot or Lowe's). Build a stage, (wood on blocks, no fasteners) hold the concert (Warning the singers to not jump around too much on stage... no fasteners) and then just load up the pieces and your receipt, and return for a full refund.

Just a thought.