what speaker is it? do you have the factory manual?
the power of the speaker has little to do with the box design, you have to look at the T/S parameters of the sub to properly determine the right box for it..
understanding T/S parameters is not an easy thing, so that's why i would let a program handle that for me.. WinISD or BassBox will use the T/S parameters of the sub and calculate an ideal enclosure for you.. (i prefer WinISD for ease of use and accuracy)