Enclosures for Car Stereo Woofers:
Because woofers move a lot of air they generate a back wave behind them. If you mount a woofer in free space without an enclosure you will get almost no bass because the back wave will cancel out the sound from the front of the woofer. There are many different types of enclosures for woofers to deal with this backwave.
A common one is a ported box.
This enclosure has the woofer mounted in box with a hole in it and a port (tube) attached to the hole. The port is made a specific size and depth to cause a "bump" or rise in the frequency response at that point. This makes the overall system more efficient but can lead to the bass to be somewhat "boomy" or less "tight" depending on how its done.
A recent technique is a bandpass enclosure. The woofer is mounted inside the box and fires into another chamber within the box that is ported to the outside. Again, this increases efficiency significantly but only at a certain frequency. This effect can make the system very loud and boomy. Another method employs mounting the woofer (which needs to be a free air type in this case) to the rear deck of the car and using the trunk as a big box.
This method is subject to many variables but can work well if done properly. Another benefit of this method is that you do not lose space from a large enclosure box.
The oldest and most popular type is a sealed enclosure. This method simply has the woofer firing into the car and the back wave is suppressed inside the box. This method usually produces tight accurate bass but is not as efficient.
Also this method typically requires a large box to work well. Finally because of the lower efficiency of this design more powerful amps and woofers are needed to play loudly. Using the specifications of the woofer you can create the type of bass response desired.
BOX STUFF
Box Material
Speaker boxes can be made of anything strong enough to not vibrate and cause losses or distortion; the most common materials are Plywood, Particleboard, and MDF (medium density fiberboard). The box material should be considered for the application by the system designer and chosen for the individual application.
Many folks seem to think MDF is the best and anything else will blow apart when you fire up your sound system, that's totally incorrect. All of these woods have places of honor in the car audio field.
Box Construction
Typically, for car audio, custom boxes are typically made from MDF and are glued together with wood glue, the box builders typically tap these boxes together with a brad nailer or screw it together. The brads, screws or nails are basically to hold the box together till the glue dries, the glue is the real strength.
Also, most installers will seal the inside seams of the box with some kind of goop or caulk, Black silicone is excellent from personal experience.
There has been recent talk that fumes from silicone are bad for loudspeakers, but this is a false claim.
Bracing the box
When the woofer moves in and out while in the speaker box a fair amount of internal air pressure is present when the speaker moves in and a negative air pressure occurs in the box when the speaker moves out, plus a whole lot of vibrations occur from the speaker moving and these pressure changes… Thicker/denser wood reduce vibration.
If the box panels are large (or thin), they can resonate enough to create hearable distortions. Bracing the box can help reduce these distortions. In a box with two woofers a center divider can also act as a brace. Internal or external bracing is easy to make and makes the box more sturdy.  
Importance of an Airtight seal

A sealed enclosure is designed to (a) keep the back wave of the speaker from coming in contact with the front wave of the speaker. (b) causes the air pressure in the box to add a suspension component to the speaker (in other words the air pressure of the box keeps the speaker from moving too far as it plays)….
A 100% airtight box is hard to achieve. However too many little leaks can cause distortion. Typically, a few small air leaks wont change the performance of the woofer like folks seem to think, in fact an air leak would have to be huge to greatly affect (a) and (b) above… The main goal we want to do is avoid air leaks because of whistles!
Common Enclosures

There seems to be some confusion about using a divider in a woofer box with two woofers… Many installers seem to be telling their customers that by having no divider the box will make more bass! I think those installers either don’t know anything about speaker boxes or are making excuses because they are too lazy to build the box correctly…!!!
If both woofers are exactly the same, the box will sound the exact with and without a divider! There is no increase at all by not using a divider…
But there are some good reasons for having the divider!
a) It braces the box!
b) It prevents the speakers from interacting inside the box…

If the woofers are putting out the exact same sound and level then having no divider wont matter. However this is not the case if there is a variation between the speakers. If one speaker is playing and the other is not playing at all, the lack of a divider will allow air pressure to come in from the non functioning speaker causing a drop in the quality of sound from the working speaker.
This would not be the case with a divider. Even if both speakers are functioning there would still be an issue if one plays less sound. The sound from the loud speaker averages down with no divider. This also can be avoided with a divider.
So basically, without a divider even a small problem with one speaker will make the whole box sound bad, but with a divider it is common for one speaker to completely go silent and the working one still sounds great!
So the wise choice is to use a divider.
How to figure box size
To figure out how large a box is measure the Height, Width, and Depth of the box (in inchees, then Height * Width * Depth / 1728 will give you the cubic ft. of the box. Usually you will be interested with the airspace inside the box so be sure to get the inside measurements and not the outside measurements for your calculations.
How to calculate angles of the box
To calculate a truckbox or one with a simple angle, measure Height, Width, Top depth and Bottom depth use the formula (Height x Width x (Top Depth + Bottom Depth)/1728)/2 to convert it to cubic Ft.
For more than one angle you will have to measure and calculate sections and then add the sections together for your total.