I have a 2x 10" Zenclosures Upfire box, and love it. Granted it may not compete with a full size box, but thats what you live with with the trunk of a Z. I just put some Poly-fil in the box and it came out sounding awesome.
Only thing I will say though is, if you're looking to lower your Z ever, if you have Eibachs or any other progressive lowering spring, your rear end will squat. Something to think about for future mods...