Just a couple of quick thoughts...
Are you likely to run around with the ski port open in the Lib most of the time?
I ask because I think that there a a couple of things to think about before the optimum size box etc...
Ski port means sedan. Sedan means that if you go with a ported box, you are efectively porting the sub box into a second sub box- the boot. Sound is air movement. Bass is lots of air movement... The only air moving in the cabin in the sealed boot senario is the panels and materials of the car its self. You might as well put a big viberator in there.
That said- it works. Just listen to all the F*ckwits in Falcodores and Lancers running around where the predominant sound is the number plate on the back rattling!!!!
If you put enough power into the sub in this situation you will get plently of bass, but inside the car it's like having the top end in the room with you and the bass coming down the hall from the bathroom, with the pictures on the wall helping out on the way...
Getting sound in the car; easy. Making it good; expensive and time consuming...
Do you have the Mac system, or are you totally reloading the system?
The standard (non premium) has not got a sub as far as I know, but even then you might be in luck with the frame work still in place...
Could you look to reload the existing sub position in the rear shelf with a long throw high power sub, and amp that?
Not as loud as the box option, but this is feeding straight into the cabin, and should work better (more musical), and take up appriciably less space...
Alternatively, push on!

your neighbours will love you and you'll look real ricey good like...
BTW- the boston does look good... If you still do the box- go a sealed box- it works better in the sealed boot senario, but still refer to above...
Whiteline rear sway, King lows, Custom quad tip muffs, Subi sport grill, HID's, Boost guage, JL slot port 12" off the standard mac, RB3l 18's ezeflash tune 150.9kw@4w 15psi 36degC...