Acoustic Treatment of 'Unique" Room

Didn't see an exact fit in any forum topic but I thought this best here...
I just rented a house and wanted to setup a high-end system apart from the HT setup in our family room. The only suitable room is our living room which is very open with 22 ft ceilings.  The issue I have with the room...

The speakers are set-up in the only position I can really use, on the short outside wall about 3 feet from the back wall.  One side wall (right speaker when looking at them is  standard drywall with a couple of small windows. However there is no opposite sidewall so to speak. The divider on that side of the room (between living room and dining room) is an open design like a stair case, I guess to make the smallish dining room seem more spacious. That structure contains a top handrail and balasters about every 3 inches from the handrail to the floor   As it's a rental I can't change that permanently. What would be best to position against the balasters to " create" a sidewall, even a low one? I have thought of using everything from a piece of birch plywood (it looks awful) to an custom sized (42" x 44") acoustic panel. My fear is that the panel while it will look more finished will be too absorptive relative the the drywall on the other side wall. However leaving it as is means soundwaves from the left speaker have no sidewall to bounce off of. I just ordered an Anti-Mode digital EQ that I hope will help no matter what I decide to do.

Unfortunately, there is no other space in the house for this system so I am stuck doing what I can to make it sound as good as I can get it.

Thanks in advance for suggestions.

Ian