Budget PA Subwoofer for tailgating

Greeting all,
My quest started over on DIYMA as I used to be very much into car sound, and my searches have led me here. A buddy of mine recently purchased a pair of cheap 2-way 12" behringer powered speakers for our modest college football tailgate. I really had no idea what to expect as this is my first foray into pro audio, but was not very happy with the complete lack of low end. I suppose I should not have been surprised given the price and size of the cabs. Anyway, now we are looking into a subwoofer. Being on a budget, I immediately thought diy. I have tools and the necessary skills to build pretty much anything, I just have no idea on designs. I've been reading and reading, but the discussions here are way over my head and I have no frame of reference on which to judge the measurements and graphs you all post of your builds. Basically, I need something portable, loud, and able to be crossed fairly high to blend in outside with the anemic cabs we currently have (which will scream in the amount of space we generally occupy). I would still like it to extend fairly low though. I would prefer a single driver enclosure. I have about 800 watts to work with and would like to maximize output/$ (preferably at or under $300 for driver and wood). It will probably mostly play rap, and pure volume trumps sound quality (though *I* would prefer something other than a one-noter for when we play other things). Portability is also second to loudness, but it has to fit in the back of an SUV along with a few other things like an ice chest or two. The first recommendation I received was for the 1850 folded horns, which look promising to my untrained eye. But the pd1850 is not readily available in the US, so could you all recommend a suitable replacement driver? Also, I've become interested in tapped horns since discovering this site as they seem to play fairly high. Any specific plans you can recommend? As a reference, my buddy is beginning to get his mind set on buying the behringer b1800x-pro to match his behringer cabs, so as long as I can exceed the performance of that I'll be happy.
P.S. available processing/eq'ing is at a very minimum...basically a high-pass and a low-pass built into the amp.

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