its always better to diy than buy a ready built one ..
i got a cheapy pair of 12's but built the box according to the theile characteristics and bloody eck did they drop well .. even put my mates very expensive ones to shame..
BUT the box was huge only just fitted in the back of the erst about 2 inches below the parcel shelf and 3 inches either side ..
plastic drain pipe makes a good porting material but dont forget to make the box nice and solid with supports for the port .. the ends can be melted into a curved shape this allows a better sound cuts need to be smooth, if just left cut it can sound a bit raspy .
if you want some serious bass then start looking at a folded port design, built some large cabinets for my old mobile nightclub set up a few years ago .. longthrow deep bass.. but size matters 2 x 12 in each box