do you no were i can get one custom built and how much they go for usually

 

The Solo 18" is 5k RMS and has 89.7dB efficiency. An Fi BTL 18", AA mayhem, etc (2-2.5k rms/94.xdB) should be louder on less than half of 5k and be a good match for a 2500.1 (if it's 2500W @ 1 or 2 ohms).

How much space you got?

 

I unno a trunk full lol

 

Get a measuring tape and figure out the maximum length, width, and height that you can fit into it, either from the outside or the inside. Don't just measure the maximum dimensions inside the trunk because the box still needs to get into it.

How are you going to fit a huge box for an 18" when you don't know the space you're using? How are you supposed to figure out the port tuning without it?