I would try putting the speakers with their backs to the solid long wall firing towards the balcony railing. I would place them about 4ft out into the room, 9ft apart, and put the listening chair just in front of the rail. That might just work!
A room doesn't have to be sealed to work well, don't worry so much about the openings except they do affect bass response and can cause an 'imaging' imbalance if you set up speakers so one speaker is adjacent to a wall and the other is adjacent to an 'open space' (there is another thread up right now on that issue.
FWIW I personally prefer the sound of speakers located on a short wall when ever possible. The sound seems more resonant/warm/natural, and on a long wall it has sounded cleaner/cooler/more clinical. But it might not be so in you home so give each wall (or open space).
A room doesn't have to be sealed to work well, don't worry so much about the openings except they do affect bass response and can cause an 'imaging' imbalance if you set up speakers so one speaker is adjacent to a wall and the other is adjacent to an 'open space' (there is another thread up right now on that issue.
FWIW I personally prefer the sound of speakers located on a short wall when ever possible. The sound seems more resonant/warm/natural, and on a long wall it has sounded cleaner/cooler/more clinical. But it might not be so in you home so give each wall (or open space).