Yeah, that Nakamichi sub is going to be next to useless in a sealed half foot enclosure. Even at .85 cubic feet, the gain drops to -3 dB by 80Hz. By 30Hz, it's all the way down to -13 dB. Practically inaudible at the levels you want a subwoofer to cover.

A vented enclosure should definitely be a requirement. But even in a vented enclosure, I wasn't all that impressed with the driver. The best (reasonable) I could find used a 2 foot enclosure tuned to about 30Hz. In that box, the frequency curve stayed pretty constant at -3dB from 30-60Hz, and climbed to -2dB by 100Hz. Not ideal, but at least it was fairly flat.

I should note that I wasn't able to find parameters for BL & Le for the driver - I suppose that could have affected the output. But according to the "how-to" those specs aren't required to get a good plot. Anyway, you can take my word for it, or check it out for yourself. The software takes a few hours to figure out, but like I said, it's worth your while.