I would go for about 2ft^3 tuned to about 34Hz. With about 20inches^2 of port. That will still sound good musically. If you don't care about sound and just want SPL then raise the tuning frequency.
i want a ton of bass but i want it to sound good to. im like leaning toward more bass tho cause the back seat in my car is xtreamly thick and kinda acts like a deading tool lol , if that makes any sence
I think a tuning of around 34Hz will give you some pretty good output without sounding terrible. I wouldn't go above 38Hz if you still want to have good sound quality.