First materials list.First materials list.
Wood for top of box. MDF or birch plywood. Preferably MDF...it shapes easier and resin will soak into the wood. Birch is lighter though.
1 to 2 gallons Polyester fiberglass resin and MEKP to activate the resin
2 or 3 3" Disposable paint brushes (these can be cleaned with acetone and reused)
Fiberglass mat (preferable mat not cloth...mat folds easier)
Blue painters masking tape
mixing cups for resin
painters masking paper
Pam cooking spray
1/2" brass air hose bushing (used to couple two air hose together)
Piece of scrap wood about 12" X 12" does not matter type of wood
3 or 4 pairs of latex gloves
Stir sticks

1. Determine how much airspace is needed for the enclosure.
2. Determine available airspace in cavity (this case the center well of the C5).
Note:Once these two variable are known you can determine what the top of the enclosure will be shaped. Some subs will require the top to protrude upwards to give more airspace to the sub. Some can be made flush. Making the mold is the same for both kinds. The difference is how the mold is attached to the top. For this I am going to talk about a flush top.
3. Prep the car. Using the blue masking tape line the entire well with the tape. Make sure it overlaps each strip and there are no air bubbles or gaps. Tape must be applied anywhere there is going to be resin. I tape another two inches around my mold area. Then using the tape and the masking paper mask the rest of the area around the well. Resin can and does fling around. You do not want to get any of it on the factory carpet.
4. Now mask the back end of the car with the tape and paper. You will have to lean over the bumer and quater panels to apply the glass. Don't want any resin on the car or to scratch the car.
5. Prep the fiberglass mat. Cut the mat into pieces and lay it into the well. You want the pieces to be like a puzzle. They should fit the counters of the well and overlap each other. There should be enough cut pieces to cover the entire well in two layers. Also cut out extra strips in various lengths and widths to use as filler pieces.
6. Remove all the pieces and lay them to the side in a place they can be easily reached while working on the car. I use a roll around cart with a paper top that I can place next to the car.
7. Now mix up some resin according to the direction on the can. Mix about a quart of it at a time. Mix the two parts very well using a stir stick.
Note:Remember to do this in an open area. Also if you are not experienced with fiberglass resin it would be a good idea to mix a little bit of it and play with it on a few pieces of mat on a scrap board first. Resin will harden faster in direct sunlight so this should be done in the shade. Adjustments to the MEKP can be made for the weather. If it is very hot less should be used and more if it is cold.
8. Place your paint brush,brass bushing,can of Pam, and bucket of mixed resin on your scrap piece of wood. Place those into the hatch close the well. Spray the area to be molded with the Pam...light coat. This is to help make the mold release. There are mold release products that can be used for this but Pam will do the job.
9. Special step here. If you do not have an air compressor skip this step. Take the brass bushing and place it at the bottom of the well facing upwards. Fiberglass all around it but do not fill it with resin. Make sure resin does not get underneath of it either. This will be used later.
10. Start at the top and work your way to the bottom. Wipe a coat of resin on the well using your paint brush. Then place the first piece of matt on the well. Next saturate the entire piece of resin until it looks transparent. There should be no air bubbles. Airbubbles will look like white spots. You can remove them with a technique called stippling. You stab the bubbles with the paint brush until they are gone. Air bubbles will cause poor adhesion and weak spots. Continue this process till the entire well is covered in mat. Make sure there are plenty of overlaps inbetween pieces.
11. Let the mat dry till it is rock hard. Should not take but an hour or two depending on the weather and how much MEKP was put in.
NOTE: To much MEKP will cause the fiberglass to dry to fast making it warp and become brittle. To little MEKP and the resin will not dry and turn into a jelly. Too little can be helped along with the sun or heat lamps. Too much and it can cause a wasted project.
12. Once it is dry it is time to remove the enclosure. This should be done as soon at the enclosure is dry. DO NOT let it sit overnight for good drying measure. The Pam will no longer work and it will be very hard to make the enclosure to release. IF you have an air compressor and installed the brass fitting. You can use an air blower to push air into the bottom of the well and help release the enclosure. This works very well. IF you did not install the brass fitting you will need to use a little elbow grease and proably some prying to get it to pop out. The first time is the hardest. Once you get it out it will slide in and out easily.
14. The inside of the enclosure should be grinded/sanded a bit with 80 grit or lower. This will make the next layer bite into the first layers. This only needs to be done where more fiberglass is going to be laid. Also it does not have be grinded smooth just roughed up a bit.
13. Once it is out you might want to make the box thicker. Depends on how much the sidewalls flex. If that is the case you can either put it back in the car and apply more glass or do it outside the car. The advantage do it in the car is it will still fit for certain. Disadvantage will be the chance of getting resin on the car again. If you do it outside the car it is very important to make sure the mix is not too hot and causes it to warp. If you add material it will have to be grinded again.
14. Once you are satisfied with the fiberglass part of the enclosure put it back into the car. Now you can cut the top. The top should fit as tight as possible and slide into the enclosure. Cut the hole for the subwoofer at this time also.
15. Cut some more fiberglass strips in 2" by 6" lengths. You will need about twenty of them.
16. Place the top into the fiberglass enclosure. The top should be tight enough to fall in. You can also screw the top in from the sides with drywall screws. Just make sure to countersink them so they do not interfer with the box dropping into the well.
17. Once the top is fitted correctly..either pressure fit or screwed in. It is time to fiberglass it to the bottom.
18. This step can be done in or out of the car. I prefer out of the car. With the top in place move the box to a sturdy work surface.
19.Mix another quart of resin.
20. Make sure you wear your gloves. Fill the gaps around the top of the box inbetween the wood and the fiberglass with resin. If the gaps are large you can put a little mat in the gap. This should be done from the top. Next wipe a coat of resin inside the box below where the wood and fiberglass meet. Now did the strips of mat into the resin. Make sure they are very saturated. Now put them in the enclosure using the them to tie the wood and fiberglass together. Do this all the way around the wood from the inside. You can go through the sub hole.
21. Another option is to fiberglass the the top of the box from the top. This can be done but once the glass dries you will need to grind the edges flush with the molded part to make the enclosure slide back into the well. Actually there are several ways it can be done and it is up to the builder and the application. Important part is the top is sealed to the bottom.
22. Now it is pretty much done. Sand off any excess resin on the top of the box and cover it with whatever you like. If you did the brass piece in the bottom that needs to be sealed up with either silicone or resin.

I think this pretty much covers it. Every application is a little different and all sorts of variables can come about. Different techniques can be used to accomplish the same goal. I have built well over 100 fiberglass boxes and hardly ever do the same exact thing twice. Hope this helps let me know if you have any questions