I've been doing some reading and studying on the use of "sonotube" for building sub enclosures. I had an old 12" Pioneer woofer here and decided to take a stab at the project. I haven't tried it in the Jeep yet, but I've hooked it up to my home stereo and I'm surprised at how good it sounds.
I already had the woofer and some MDF and 3/4" plywood scraps. A 4 ft length of 12" diameter sonotube cost me a bit over $6. Took about 2 hours to make the darn thing. Stands about 24" tall, is sealed up tight. They can be either sealed or ported with good results. This is a passive enclosure, so not like the Bazooka enclosures that are powered.
The real test will be when I can put some dedicated power to it in my Jeep. Just thought mentioning it might gives others here an option if the budget is tight.