Here's the finished product, a Dayton Audio 10 inch sub in a ported box about .9 cubic feet.
My initial plan was to put my old JL Audio 10w6 in the M, but when I fired it up, it was making a weird noise, like the voice coil was stuck or dislodged. Must have happened when I moved, as I've not used it in a car for maybe 10 years (My G35 had a JL 15w6 in it, but that's another story)
Here's the dayton sub (right) alongside the JL sub (left). There's a guy on ebay who apparently replaces the guts of the JL subs for maybe $100. I may try that some day but wanted to give the Dayton a shot, since it got rave reviews at the DIY Mobile Audio site, and was just a little over $100.
First off, I taped up the location where I wanted to build the fiberglass box. Use lots and lots and lots of painter's tape. I'm sure there are other methods, but this worked for me. You don't want any fiberglass resin to get on your trunk carpet. Mask off everywhere else, too so drips don't get on your paint, metal, etc. I also sprayed the tape with cooking spray to ensure it doesn't stick to the fiberglass later.
Next I cut the flat base out of particleboard. You can use MDF if you want, but in the end for such a small box it makes little difference. Once the base was in, I mixed up some polyester resin and painted it on some fiberglass mat, and put it in place. The upper layers were tough since I had to basically use tape to get it to stick upside down. Once you get the initial layer down, the subsequent layers will easily stick to the dried or wet fiberglass. As a side note, the stuff I primarily used was some sort of fiberglass cloth backed with fiberglass mat. There's some sort of trade name for it but I don't remember what it is, I just bought a giant box of it years ago from someone on ebay. You can use either/both, makes little difference in the end, as long as you use enough layers. I've even seen people use grill cloth or fleece to build speaker boxes. Fiberglass also bonds easily to wood, in case you were wondering.
Here's my frankenstein looking enclosure progress. Still need a few more layers. I'd say you need to go roughly 3/8 thick to allow for the speaker, unless you have a monster sub with a million watts going into it.
I wanted to see how much volume I had so far, and a bunch of mega-bloks were handy so I filled it with that. Some people suggest packing peanuts, but those are messy and I didn't have any around. you can basically use anything transferrable, even water as long as your enclosure is indeed water tight - which it should be but won't always be 100%
Speaking of water-tightness, I poured some in my box and found it was severely lacking in that department. so, as per a suggestion on the fiberglass forums (I wish I remembered the site, DIYMA.com turned me on to it, I'd cite them if I could), I mixed up a batch of resin and bondo, and poured the soupy mix in and sloshed it all around in the crevices to try to seal things up, and add a little lateral strength.
The front of the box and the requisite port presented a challenge, but here's the beginning of it:
Here's the backside of the front, showing the 4 inch PVC pipe I used for a port, as recommended by the Dayton Audio site. As a side note, I bought the 4 inch 10 foot long pvc pipe at Lowe's on a very hot day, and upon sliding it through the ski hole, it bumped into my windshield and CRACKED THE WINDSHIELD so be careful. This added $500 to my project.
Here's the front attached to the rear portion I had previously built. I just put layers of fiberglass inside to the extent I could reach, and then some layers on the outside.
