Well, there are a few guys on here who didn't go with the JL setup and designed there own to fit in there. I would look and ask around.
You could probably build a pretty decent box for around $50 of materials, and in a sitaution where you are not looking for heart stopping pounding bass, and just fill in noise, you could go with 1/2" Particle board to try and cut some costs. It would also allow you to have more internal dimensions then using 3/4". And it weighs less than MDF too...
I think I saw someone design a box entirely out of fiberglass back there. I mean, there are ALL kinds of ideas that you can come up with.
Customization is king...
If you have ever been to Tuner Bash, you can find out that the most customized will beat the most expensive probably 8 out of 10 times in a car show.