I would say symmetry is the key.
The actual arrangement that you have now with the speakers at the low end of the ceiling is likely the best you can do. Quite possibly, with both speakers having corners, the bass from them may well be quite even.
There seem to me two obvious problems: the window, and the side walls.
The window will reflect sound differently then the sloped ceiling. Perhaps a temporary cover for the window made of the sloped ceiling type of material, or perhaps a mirror/pane of glass above the left speaker.
For side walls, the left speaker will give reflections off the left wall whereas the right speaker gives no side wall reflections - this is for frequencies approximately 200Hz and above. The solution would be either some absorption on the left side at the reflection point, or some reflective material similar to the left wall placed at the reflection point by the right railings.
These problems should be dealt with first as otherwise I can't see how you will get a decent sound. After that you can move speakers and chair around to get the best sound.
Generally you may find it better to bring the chair out from the back wall a bit. If you can place your speakers and chair anywhere then try a range of positions until it sounds the best. Use large movements - say one foot - at a time, and then home in using smaller increments. Finally try some toe-in.
Be patient. It is frustrating work but should be worth the effort.