1. For speed and efficiency, the sides of our enclosure are constructed using MDF rather than fiberglass.
With the bottom fiberglass shell of our composite enclosure cured and ready to go, we need to start on the side walls. Although building our enclosure entirely out of fiberglass is possible, we will construct the vertical sides of the enclosure from medium density fiberboard (MDF) for both speed and efficiency. Taking advantage of the air space in a curved or irregularly shaped area is one of the main benefits of using fiberglass instead of MDF when building an enclosure. However, it doesn't make much sense to lay up fiberglass on the sections that can be easily built with pieces of MDF. The multi-composite enclosure combines the strengths and benefits of fiberglass and MDF.
THE FENCE
For this project, the sides of the enclosure will be a simple rectangular "fence" of 1/2" MDF. This example is simple because the fiberglass shell is flat and level around the perimeter where the MDF will attach to the fiberglass. With most projects, this would not be the case and the bottom edge of the fence would need to be cut with a jigsaw or bandsaw to match the contour of the fiberglass shell. Using cardboard and tape to make patterns would aid this process. The bottom edge doesn't need to fit perfectly to the shell, but any gaps should be less than 1/2". The 1/2" MDF sides were assembled with wood glue and brad nails. One-half-inch MDF works fine for small speaker enclosures, but 3/4" MDF should be used for most subwoofer enclosures. The top outer edge of the MDF frame is notched with a rabbeting bit on an inverted router table. This important rabbeted edge allows the top of our composite enclosure to be fabricated with maximum strength and durability (more on this later).
2. The bottom edge of the MDF "fence" is roughed up with a 2" 24-grit grinding disc attached to a right-angle die grinder. This increases the bonding area between the MDF and the fiberglass shell.
The MDF frame is placed on the shell and traced with a permanent marker where it will be attached. The fiberglass shell is roughly trimmed 1/4" larger than the outline. This can be done using a die grinder with cut-off wheel and/or a jigsaw (set to a slow speed) with a metal-cutting blade (a blade designed to cut metal, not just a blade made of metal). Use EXTREME CAUTION when trimming the fiberglass because it can be very difficult to securely hold the shell while trimming the edges. Due to the fiberglass particles flying in the air, thick protective gloves, eye protection and a dust mask are a must.
Once the shell is trimmed, fiberglass-reinforced body filler bonds the MDF frame and fiberglass shell together. The milled fiberglass fillers in these body fillers make them stronger and warp less than standard body fillers. With their thicker consistency, they also stay where you put them and don't sag. To properly mate our fiberglass shell with the MDF frame, the edges of the MDF where the two meet are prepped using a right-angle die grinder with a 2", 24-grit 3M Roloc grinding disc. For those of you without access to an air compressor/air tools, a coarse rasp/file will also do a good job. Roughing up the edges, quite severely, increases the bonding area where the fiberglass-reinforced filler joins the MDF and fiberglass.
3. A closer look at the roughed-up edge.
To insure a rock-solid bond, brush a coat of resin (mixed with MEKP, of course) on the edge of the MDF to serve as a primer, because some reinforced fillers will not properly bond to raw MDF. So that there aren't any soft spots when it cures, thoroughly mix up a batch of reinforced filler with the proper amount of cream hardener. Apply it to the MDF frame and place the frame onto the fiberglass shell. Use a plastic body filler spreader and/or gloved finger to evenly spread the filler around the inner and outer perimeter where the MDF and fiberglass meet. Cutting a plastic spreader to a custom shape will make application easier, while keeping spreaders in a cup of acetone will make cleaning them easier. Properly mixed reinforced body filler should harden in 15 to 20 minutes. Once the filler has cured, the outer edge can be ground down with the 24-grit Roloc disc and block-sanded flush.
4. The outline of the MDF fence is traced onto the fiberglass shell with a permanent marker and cut 1/4" larger with a jigsaw using a metal-cutting blade. Use extreme caution when trimming the fiberglass shell! | 
5. The MDF is bonded to the fiberglass shell using a fiberglass-reinforced filler. The filler is applied to the inner and outer perimeter of the fence to ensure a full-strength bond. | 
6. Two MDF rings are made using the "Perfect Circle" router jig and glued together. The top ring will allow the speaker to sit flush with the surface of the enclosure. |