Basic Box Building Information.

Check with manufacturer or dealer for appropriate box volume and design.  Some subs can't be used in certain types of boxes, and have very small tolerances for box volume errors.  If a sub is installed in a box larger or smaller than what is supposed to, it will sound bad and could be destroyed.  Boxes can be built in any shape, but it is difficult to calculate volume for complex shapes.

Materials

A box has to be very rigid.  Most common building materials are 5/8" or thicker particle board or medium density fiberboard.

If building a box with Plexiglas, do not use anything less that 1/2 inch thick.

A common material used to mold complex shaped boxes is fiberglass, but it is a real pain to work with, and several layers need to be applied for a solid finish.

Gluing, Sealing

Use glue at all joints (cheapest and most used product is Liquid Nails).  Make sure there are no holes.  Any leaks will degrade the performance of your subs, not to mention the annoying noise air makes when being pushed out of a small hole.

Let glue cure for at least 24 hours before mounting the woofers.  The fumes of some products will eat up rubber and other materials subs are made of.

Holding Joints Together

Screw joints (use 2"  -  2-1/2" screws) every four inches or so. Pre-drill about 3/4" deep, so that screws do not split the wood at the edges, especially when working with particleboard.

A box for Each Sub?

Even though it is not necessary to have two separate chambers for two subs, it is best to take this approach for two reasons:  First, if one of the subs dies, then the volume of the box will be "twice" as big, as seen by the sub that is still working.  This could cause problems and even damage the other sub.   The second reason is bracing.  building a box with a divider in the middle will be much sturdier.

Making Ports

There are several way to build ports.  If a pre-made port is not available, the most common material is PVC tubing.  PVC tubing is very rigid, comes in different diameters, and is easily found at any hardware store.

Cut the tubing at the desired length.  Consider the volume the port takes up when calculating the box volume.  Cut a hole in the box.  Make sure the hole is as perfect as possible to minimize gaps between the box and the tube.   A couple wood braces can be added for screwing the port top the box.  Seal the gaps using plenty of Liquid Nails or similar product.

Bracing

Boxes that are more than a foot on width or length or height, should be braced (use a piece of wood maybe 3 or 4 inches wide across the box, so that box does not flex).  It is a good idea to put wood blocks on the corners for reinforcement. Always consider that blocks, braces, neon lights, etc. inside a box take up space and should be accounted for when calculating internal volume.

Damping/Filling

It is advisable to put damping material inside a box.   Pillow polyfill and fiberglass insulation are common, though polyfill is a lot easier on your skin.  This increases subwoofer efficiency by dissipating some energy that affects the sub, particularly the voice coil.  Polyfill also "fools" a sub into thinking it is in a bigger box.  Play around with different amounts of polyfill until you get the desired results.

Finishing the Box

Add wood filler to holes and sand the box to make a smooth surface.  If you are painting the box, It is a good idea to apply primer under the paint.

It is not necessary to sand the box if you are using carpet or padding under vinyl, since the thickness of the material will cover any small imperfections.  The best way to cut carpet or vinyl is with a good quality carpet knife.  Blades wear out pretty quickly, so buy a handful.  Cut a piece of carpet (or vinyl) big enough to cover the whole box.  Apply adhesive to both box and carpet (spray 3M adhesive 77 or 90 works great). Wait about a minute and place the fabric over the wood.  For a good fit, stretch the fabric when applying it.   The fabric should wrap around and end in a place of the box that will not be seen.  Do one side at a time, cutting excess carpet.  If possible, add staples to hold the fabric at the ends.