When I looked into subs, I also wanted to maintain as much trunk space as possible. I made a box that sits directly behind the rear seats without taking up much trunk space at all. I had to modify the design in order to make it hit as hard as possible, and still maintain it's small size. Below are a few pictures. Notice the box is ported instead of sealed. At first it was sealed, and it had very weak bass. The port allowed the woofers to breathe. also notice the corner protrusions that maintain a 2" distance off of the back seats. You can also see the port from the speaker side in the center. The port exits the bottom of the box, then travels up the front face, then re-enters the top of the box, then exits at the rear. Also notice the box length is not long enough to fill the opening completely. This allows the bass to also flow back around the ends. After finalizing the design, it hits pretty hard. It delivers a very punchy bass, that when turned up pretty loud, tends to give me a headache. It's more of a sledgehammer type bass. The subs are Kenwood 10" shallow subs, model KFC-XW1000F. They are some of the best shallow subs I've heard. I have the shallow 10" Rockford Fosgate subs in my 2012 crew cab F150, and while they make a deep bass. They don't hit as hard as the Kenwood subs.