Crutchfield has a JL Audio Single "10 sub in an enclosure specifically designed for the MINI......but it's not cheap!!
I tried to post a link to the product for you but I'm not allowed as I don't have enough posts, but you can find it at Crutchfield yourself. Just go to Crutchfield and search for products that fit your car, it's the only sub listed.
I'm building my own enclosure when I have time for 2 subs but I'm also removing the back seat as it's pretty much useless for adults anyway.
I'm always willing to learn......if someone is willing to teach!!!