That isn't a bad place to put a sub at all. I don't even think you will need a powerful sub because you are seated nearby the sub, and better yet, against a wall, where bass tends to be the strongest. So don't let anyone tell you you need a 15" sub, you really don't (of course, it doesn't hurt though). If I were you, I would take some measurements to see what can fit there, and then look at the offerings from Outlaw Audio, Hsu Research, and SVS. Maybe also Elemental Designs if you are not in a hurry for the sub. Also Epik subwoofers if you want massive overkill. Don't worry, despite the openings, you are not in a bad position for a sub.