I have an engineering degree.
Determining the port tube diameter and length for a sub box is very complicated.
You can replace the 2 12' speakers with home audio stlye 12"bass speakers. If you wire them in parellel it will reduce the ohms say from 8 ohms to 4 they are cheaper and sound great. ive seen it done with radio shack speakers
If you want to use one 12 make a new sealed front plate then you can experiment with different port sizes and lengths,
do do it right you need a good sub cross over rolled at say 80-100 htz with a roll off rate of 12-18 db per octave.
then a specrum anaylser will help to tune the sub in the car.
Hope that helps If not just memorize what i said and repeat it at stereo shops and they will think your a whiz.