Not sure if everyone has seen this, but there's a guy making speaker boxes for the 370Z That look reasonably nice and selling them on Ebay:
It's $170 with shipping and can be ordered for one 10" speaker, two 10" speakers or one 12" speaker.
I though about buying one, but there are multiple things I didn't like about the design:
1) The box is too tall and partly obscures the view out the rear window.
2) There is no space to mount an amp onto the box in an attractive fashion, which means you have to find another spot...and take up more of our limited space....for your amp.
3) The rear facing speaker would be vulnerable to damage from anything in the cargo bay (even with a speaker grill). After all, this is a sportscar and under hard braking, stationary objects can become missiles.
4) The carpet is ok, but I wanted something a little more upscale looking.
Anyway, it's a reasonably priced solution and my custom box was in the $350 range.