I have never put a "system" in any car I've ever owned, maybe switched out oem head unit for aftermarket, but that's about it. A few years ago I switched out my Bose system in my 95 convertible for a pioneer single din head with 6x9s in the back and 6" rounds in the front. It never really sounded good, but that was secondary to top down driving! About 3 years ago my friend gave me a 12" sub, so I bought a nice little 600 watt amp and never installed it. Life sometimes gets in the way.
A few weeks ago I finally took the time to install everything. Maybe a 12" sub is overkill for such a small car, but it's what I had, sooooo.
Like I said, super simple design and install. I thought about taking the compartments out in the back and lowering the sub into the compartment behind the passenger, but it was more modding than I wanted to do. Also, this design can be disconnected and removed rather quickly.
I used a 2'x4' piece of 1/2" mdf. I couldn't find anybody locally to source cover material for the box, so I ordered some online . I was hoping for a closer color to the carpet, but what I found was as close as I could get.
Finished, the sound , for me, is awesome. I won't win any design awards with it, but I sure adds depth to the sound.

This was after initial install, just setting everything up.

The back of the box. Super simple. It just wedges into the back and rests on the top of the area behind the seats.

The material resting on box. I used spray adhesive to hold it in place and staples on the back to hold the edges

How it sets after install. Like I said, not all that purty, but it functions like you wouldn't believe.
The top can still be stowed and with the top up I can carry a few bags of groceries!
Thanks for looking.

 

Is the back of the speaker open to the air? If so, you've got an effectively 0 baffle setup, and are getting hardly any of the bass its putting out. Make the backside into a sealed enclosure, you won't believe the difference.

 

Agreed. The speaker uses the enclosed volume of air as an air-spring to deliver a pressure wave into your cabin, for face-melting low's bro

 

The back of the box is solid, the front is open. The box uses the front of the trunk area that right behind the seats to finish the enclosure. The box wedges into the space and up against the front of the trunk area, effectively creating an inclosed box. And believe me, it works very well. When you turn the bass off, it really makes a difference.

The back of the box

The front of the box lifted to show how is sets

The front as it sets in place. It makes a nice little box for some good base. I'm not interested in face melting thumps, just something to fill out the sound.
Thanks for the input though. I hope it makes more sense now.

 

That probably seals up good enough, but I would finish off the enclosure with mdf personally. Any air leaks there will make the bass less tight and more sloppy (not great descriptions I know). It will just have a better sound quality fully sealed.

 

If it is not a free air woofer you are working it real hard.

 

Seal up the enclosure and you will get a lot more output from the sub.