I have to agree with you after seeing that photo you posted, that space looks awfully small so doing 2 ported will be a stretch and be either be smallish airspace or port area IF you find a way to fit it.
If what your lacking currently is decent lows I'd suggest a single Sld 10 ported. It won't gain across the board but will definitely put the emphasis down low (fatter lows) if that's what you're lacking currently. "A different sound so to speak"
I've seen others ask for dimensions and am curious as well, you said there's some BS back there to build around and that sux but nothing is impossible unless it's IMPOSSIBLE... lol.
Personally I'm not a fan of sealed box subs no matter the manufacturer, the decent ones always are accompanied by "esoteric" pricing and that negates the performance for me. I think as long as your expectations are realistic a single ported 10 may do the job. I truly dislike the peak most 10's produce in a small sealed box.