Sub for a 200

[DealsGapCobra]

I spent a few hours yesterday trying to design a sub box for my 200 but can't seem to find a way to pull it off without losing far too much storage. I am now considering doing an install like in the Sport 200 with a Polk MM (maybe 12" ?) mounted in the walk-thru. Have any of you listened to this system and is it reasonable? I am not looking for serious bass but the stock 6" polks definitely need some help down low.

What size amp should I be considering to drive the sub?

Thanks in advance.