I dont think you have enough depth in that room to run 7.1, I would personally stick with 5.1, and do like St_Patguy reccomended, place the left (as facing the chairs) surround on the back wall, or hang it from the angled celling. the axiom QS8 mounts, can adjust for the angle of that back drop.. Also, i would bring the right QS8 to the same plane behind the chairs, so that both of the surrounds are the same distance from the chairs.
Your current setup kind of reminds me of one of mine... I had a QS8 on 3 Arizona Tea cans that i was using as a stand for a couple years... the unopened cans worked quite well actually.
