Something tells me you're jumping into the deep end of the ocean with horns... By far, they're probably the most complicated enclosures to design and build. If you want to take a stab at a design for your sub, try visiting
http://www.users.bigpond.com/dmcbean/ and download Hornresp, which is a horn response analysis program that will predict your output and much more based on the driver, pathlengths, etc etc. I'm not sure if it supports folded horns or not... At any rate, good luck with that... The pathlength on a typical 15-inch sub would be rather large, so designing a horn to fit in a vehicle would be a challenge, to say the least...
If you visit
www.billfitzmaurice.com and look up the design plans for the Autotuba, you'll notice his folded horns allow for all the benefits of having a horn-loaded sub in a small package. I'm actually in the process of building one myself, and based off the results I got from the Tuba24, I think this thing is gonna pound... It uses a single 8" driver in a folded horn design with final measurements at 14"x14"x32". Considering the driver itself is only worth about $30, and it supposedly outperforms two sealed 10" subs, I think it's pretty amazing... Anyhow, check out that site. There are some great designs, and they're fun - although challenging - to build.