the open 2 sides will play havoc with the stereo balance - you need the walls to the sides of the speakers to be similar if not identical. you might be able to help things enough to be tolerable by building a reflecting panel/wall segment to place at the side of the speaker that lacks a side wall.
(I used to live in a loft space that had this issue, although with different variables than yours)
(I used to live in a loft space that had this issue, although with different variables than yours)