Depends on what kind of music u listen to, the equipment, how much trunk you want to lose and how much weight you want to add. I personally use to work in the car audio field and built a variety of sub enclosures. I like a ported box, but they are usually on the larger side of the spectrum... EXCEPT for the Boston G1 or G2 if you can find them local. In a 98 Cobra I build a Ported enclosure tuned at 45hz for a Boston G2 powered by a Boston GT-22. WOW is all I can say. The boxs total vol was 1.3cuft. That's TINY... It wasn't much bigger then a truck style enclosure... It also had a sqrared port. It was Very musical and got down and dirty..
Pre-fab boxes are ****** and don't rec them UNLESS you are on a budget. Spend the coin and get a custom tuned box. Easiest way to explain it is this... Say you wear a size 10 shoe... Go run track in a size 8 or 12. Sure at 1st ur like ok, but within NO time your feet will hurt taking away your potential to do any good.... Same with speakers... A solid box is only PART of the equasion... Source and signal is a MAJOR factor too. Match your equipment up Properly... RMS to RMS. I love seeing the junk amps advertise 1000watts then you look at their RMS being 50x2 @4ohms hahahaha... 1000 watts would happen IF lightning struck it and POOF.... Toast.
That's just my $.02
Hoping to save people from frustration one at a time