This step by step shows the process to build a basic sealed box.
In my case, I picked up a sub/amp/box combo on Craigslist for pretty cheap, however the box was slightly too big to fit into the trunk of my 1997 240sx (only a 15" opening). So, motivated to make my purchase work, I decided to build a box: Spending less than $50 for the materials.
Step 1: Planning: Figuring out cubic feet
In this case, I was using an Infinity Kappa 12.1 which recommends a .75 - 1.25 cubic foot Sealed box. Reading some reviews, I chose to go with 1 cubic foot. (sealed vs. ported depends on your preferences. Sealed gives you a harder/punchier bass, ported gives you a wider range and can be heard from further distances. Read up more on sealed vs. ported if you're unsure on what you would like)
To figure out the cubic feet of the interior of the box you are designing, you multiply Length x Width x Height and divide by 1728
In this example, the exterior of this box is 10" x 16" x 16"
Keep in mind the thickness of materials - 3/4" on each side = 1.5"
8.5 x 14.5 x 14.5 = 1787 Divided by 1728 = 1.03 cubic feet
*Depth of the box should be judged by the depth of the subwoofer + 2" minimum*