How to Make a Fiberglass Subwoofer Box


Fiberglass subwoofer box isn’t easy DIY to come by, but they come in handy once fixed. The subwoofer holds lots of benefits for car audio setup and if properly installed, can make your car audio system standout and unique. 

I’ll be taking you through a step-by-step process on how to make a fiberglass subwoofer box in the trunk of your car. It might seem a bit long process but trust me, it’s easier than you think so, get back those negative hunches. 

Here’s a list of materials needed for our premium fiberglass subwoofer box:

  • A gallon polyester fiberglass resin
  • Resin hardener
  • MDF and respirator
  • Fiberglass matte
  • Dremel and paintbrushes
  • Tinfoil, masking tape, glue gun, and wooden dowels
  • Fleece fabric, light sandpaper, and packing peanuts
  • Carpeting and wiring materials

Making A Fiberglass Subwoofer Box

, How to Make a Fiberglass Subwoofer Box

This new project we are about to embark on will take about 8 hours to finish, together with the adjustment and drying time. We’ll go through 12 different steps for the procedure so, heads up and get your tools ready!

Empty Your Trunk!

Right, we need your trunk emptied in order to get started. This is to ensure that nothing gets in the way of the job which can slow down or pose unforeseen issues later. However, the carpet should be adjusted smoothly to every corner of the trunk. Again, ensure the space is thoroughly clean and free from debris.

Layer Tapping

Simply create a layer and tape the area you have decided to glass. It is advisable to tape two layers to avoid resin seeping through in the long run. Also, ensure the tape layer goes past where your box will size with about three to four inches.

Tinfoil Addition

This might not be essential but for the sake of keeping the resin intact, the need for it might be necessary. Make the foils into squares, tape as you go, and add newspaper around your work area.

Resin and Fiberglass Layer Addition  

Ensure to wear a mask during this step of the process and that the environment is well ventilated. Mix the required amount of resin and hardener in different small batches in a plastic cup; about 2 to 4oz at a go.

Next, you prepare the fiberglass by ripping the matte into about 1 x 4 inches long. After this, you can start with the layer procedure. Dip your brush into the resin and spread it evenly over the area for the job. 

The job has pretty much started from this point on, take from your glass strips and place them on the surface, add resin onto the strip the brush once again. After this process, lay the fiberglass to cross over the other pieces till the tap[ed area is fully covered. Note: Ensure your shell is bigger than your intended size because of later trimming.

Fiberglass Removal

Once your fiberglass is strong enough to be removed, add about 4 to 5 layers of glass on the inside. This can be done inside or outside of the trunk, this is totally up to you. However, if you are working inside of the trunk, ensure to keep the contours of the trunk in the box and that it is strong enough. Regardless, you will still need to move the glass out of the trunk to get it strong enough. You can then move it back once it’s all dried.

Fiberglass Excess Trimming

The excess edges are inevitable, so you will need to trim to have clean edges. Do this easily by drawing a line where you want the edge to be. Take your Dremel or other related cutting device and carefully work along with that line. Since it’s well dried, the thickness shouldn’t be tampered with even with the cutting. 

Ridding Obstructions 

Some vehicles do have moving parts in their trunks which could pose a problem when it comes in contact with the box. These obstructions should be noted and gotten rid of before they pose a bigger problem. The best time to rid of this is before installing the speaker rings. You can easily do this by creating a jig of the MDF for a suitable pocket in your box. Look out for where an obstruction might occur, use a wooden dowel together with hot melt glue to establish an anti-obstruction. Your speaker rings can follow once this is in place.

Speaker Ring Construction

The speaker rings are constructed out of MDF around ¾ inches. Follow the diagram manuals to cut your sub in sizes according to their specifications. Ensure the two rings are secured together even when countersinking to your sub. Either way, make the rings bigger than the normal size to allow the carpet to go underneath the subwoofer. You can browse more online about speaker ring placement.

Volume Dictation and Positioning

The size of space you have left for the sub in the speaker box will dictate how much volume you will have as this will determine the size of sub you will be using. It is always better if your box has more than enough room for other advantages. Place your speaker ring in the direction you want the angle to face, maybe upwards or the front of the car. Ensure the ring is firm and work around it with the dowel rods and hot melt glue.

Fleece Addition

Fleece is needed to stretch over the opening between the speaker rings and fiberglass edge. The hot melt glue can be used to attach the fleece to the fiberglass area but ensure it stretches out neatly. Paint the fleece with resin with your cheapo brushes. The whole area should be neatly covered except for the little space for the sub. Leave to dry and become solid.

Add up to four to five layers of fiberglass to the fleeced space and not up to the subs space. Trim the fleece that goes past the edges and cut open the holes for subs.

Wiring Subs

Some sub boxes have ready-made connections that you can simply connect wires to, if not, you can easily install one by yourself. You can simply drill a hole and work the wires through it which you seal up afterward. In case of a few humps, sand the areas with humps lightly and carefully while you protect your face.

You can finish up your box to your taste after this. You can paint the box with your preferred color and add an interior matching carpet.

Sub Installation

Your sub can now be wired up and installed in its hole with a nice and perfect fit. A sound deadener isn’t necessary but you may want one placed to get the best out of your sub and to prevent resonance or distortions. Simply run the wires back to your amp and ENJOY!

After completing all these processes, you are good to go with your Fiberglass subwoofer box.


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