The first thing you're going to need to do is look up your speaker to determine the size of the enclosure it needs. Start on the speaker manufacturer's web site and look up your particular driver. Then do a google search on building a speaker enclosure - there is a ton of good (and not so good) info out there.
Naw! This will be used in Houston. The purpose of these loud 15" speakers in Houston is to help the driver detect and show off to other surrounding motorists how loosely fitting all the body panels are in that particular vehicle. Most of these vehicles with these speakers sound like they have been down 150,000 miles of bad road.
I have built boxes for a 10" subwoofer for two of my cars. As others mentioned it starts with the sub manufacturer specs for enclosure volume. I used 10" for space considerations and the fact that I listen to music. I just want to fill in some of the missing music.
Part two of that is the proper enclosure for a big woofer is a big cabinet. Losing over a cubic foot for my sub box is enough space lost for me.
Part three is you need an amp to power the sub. I could get a nice bridged amp for relatively cheap to drive the small 10". I would guess a quality amp to drive a bigger sub would cost more money. There is more cost to a free sub that the sub.
The sub manufacturer specs .75-1 cubit foot interior for my sub. Interior length x width x depth in inches divide by 12 x 12 x 12. I add 3/4" for material thickness on some parts to get cut sizes. Some panels do not have anything added.
He would be much better off with a couple of long-throw 8" drivers, They'll be much quicker and won't 'beam' as much as a 15 would. (lambda=.5D) Music at low frequencies is out of the question anyway as there is not enough room in a truck cab to develop wavelengths longer than the cab's maximum dimension. Let's say that dimension is 6 feet, then the lowest full wave you can develop is approx 190 Hz. People who insist to go 'flat' down to 20Hz, need a 60' room. There is a difference between a tactile 'thud' and a tone. Those kids in those loud cars are listening to thuds, not notes. Just pillows of air smacking around their ear-drums and internal organs.
The other problem is going to be where the 'centre' channel bass info is going to come from. If his pre-amp has a subwoofer output, then he's all set to listen to the vibration of all loose parts inside the cab.
rec.woodworking blasphemy aside, here's a link to the DIY Network's Tricked-Out episode where they build an acrylic speaker enclosure. The site includes a link to a enclosure volume calculator.
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