jl audio's website has specs on the box you should build.
If the dimensions aren't good for your car, experiment until you get external dimensions that give you the same airspace as the recommended enclosure on JL's site.
I used to have a pair of 12W3v1, and a pair of 12W3v2, both on sealed boxes they both sounded great. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif