Do you have the stock SW enclosure? If so, there are a couple threads in the audio section where people used the stock enclosure and mounted a 10" shallow depth SW in there.

Me, I mounted 2 JL 6" subs (yes, they make them that small) in the box. It took some nibbling with the tin snips to get everything to fit, but they fit and they work great. Not 20hz great, but they play every note in London Grammar's "hey now" with no problems, all run off the stock DSP amp.

__________________
Kirk
Las Vegas
2016 X5 40e Mineral White/Black Dakota Leather, ZLL, ZCW, ZDA, ZDB, ZPP, multi contour seats, rear side window shades, HK stereo
2011 E90M3, 6-speed manual