I actually did just that in my 1991 grand marquis LS. I went with a sealed style though. It opened up a lot of space in my trunk. Of course the downfall is not having a spare tire anymore. The entire box is in an "L" shape, the box can fit 3 12" subs. I only have 2 at the moment due to lack of funds for an amp. The subs fire back into the trunk with most of the enclosure on top of the gas tank. With this setup, subwoofer grills are an absolute necessity. That's just a rundown of the setup.
When making such a box, you need to of course do a lot of measuring and prep time before you can start cutting into the wood.
Keep in mind that the filler tube for the gas tank is on the left side, and the manufacturer molded around it inside the trunk, if your going for a box that will span the full width of your trunk, then that "bump" will be in your way. I simply beat it flat with a hammer, then used silicone to seal a small crack. It didn't interfere with the filler tube at all, I still have a few inches between it and the trunk.
There should also be small brackets that are in place to hold the bumper jack secure, I got rid of them with no intention of using them again, make sure you leave the bracket that the bolt goes through in order to hold your spare tire down, you'll want to use that to hold your enclosure in place. The original bolt will have to be replaced because its too long. I went to the hardware store and bought a bolt with a ring on the end (like the ones used on a clothes line or a cheap swing set) and bent it into the shape of the original bolt.
After making your decision on how many subs your using, make your measurements, and get to building. If you need more info, email me. "
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