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*
Step 2: Materials
3/4" MDF boards - pretty standard unless you are using a much larger or multiple subs
Circular Saw or Table Saw - or have Home Depot / Lowes cut them for you (make sure they stick to your measurements!)
Straight edge / Ruler
1/2" - 1 3/4" Sheet Metal Screws
Wood glue or Liquid Nails
Carpet - if preferred (found at auto parts stores / hobby stores)
Spray Adhesive - if preferred
Speaker Terminal Cup (found online / eBay)
Compass / Circle Template / screw, pencil & string combo
Box Cutter / Razor
Step 3: Marking Your Edges
I used the boards to mark out a 3/4" edge on the piece I will drill through. This way I can stay within my limit and make sure all my screws are center.
Step 4: Pilot Holes
Step 5: Finding Center
I needed an 11 1/4" diameter circle. So I also made cross marks between my "X" and measured half that from the center point. I marked those measurements on my cross lines. (sorry, no picture)
Placing a nail in the center point, tying a string around the nail and to a pencil, I used those marked cross points to guide me as I created a perfect circle. (sorry, no picture)
*If you can find a template, this process will be much faster!*
Step 6: Cutting The Circle
I used a jig saw. By drilling a large hole inside the circle I just drew, I can get my saw blade in and start cutting. It's helpful to have someone hold the board down or clamp it. (sorry, no picture)
In this photo, I have my bottom 3 boards already drilled together, with my top piece just placed on top for a mock up.
(If you notice the right side, I needed to pull that board back while I drilled in the top piece so it could line up perfectly. Not an issue)
The same process can be used to cut an opening for your terminal cup, or even a large circle bit for your drill.
Step 7: Assembled & Carpeting
I lined the inside edges with the silicone caulk and let it dry (can take up to 24 hours)
Now for carpeting. I picked up a roll of grey carpet at an auto parts store nearby for $10. Luckily, it lined up perfectly so I could center the box on the mat and have the side fold up to the edge. A larger roll may be neccessary for bigger boxes.
Using 3M spray adhesive I sprayed the entire back of the box evenly, being certain to cover all the edges. I also sprayed the backside of the carpet that will meet with the box. Continued this process by spraying both box/carpet, then flipping the box forward. (having a large enough sheet of carpet makes this easy) Pat down the carpet and make sure its smooth as you go.
Don't worry about covering the holes, you will cut those out later.
When you get to the final side and the carpet edges meet, use a razor blade / box cutter and cut along the edge, trying your best to make it seemless. (this is where making sure you glued all the way to the edge is important)
Same process goes for the sides. You may have to cut some extra material off before you start, but make sure your material reaches the edge, spray the box and carpet then apply. Use your razor blade along the edge to cut off the excess.