I dealt with much the same situation and thought I'd end up having to do something like this. I overcame it through the simplified Sumiko method of speaker placement. In this method your speaker near a side wall is the anchor. Once you get it placed you start moving the other speaker out away from back wall incrementally until it 'locks' in with the other speaker.
Acoustic Curtain for listening area
Hello all ....I'm polling everyone who can recommend a super heavy duty non reflective curtain for the left side of my listening area which opens up into a large open space / dining room and reflects ....Seems like all the stuff on Amazon/eBay is cheap Ikea crap....any help appreciated . Thanks
12 FT Width
7.5 Height