building ported sub enclosure

ok so i know a good bit about car audio wiring ect... my weak point is sub enclosures
i have 2 12'' ipipe DVC 4 ohm subs and getting a amp to run 1 ohm.. i have a 99 chevy silverado ext cab and i plan to make a box to go on my back seat ( only the driver side ) where it will go under the seat come up and over the seat to basically sit in the seat . but i want to hit low freq. where should i put the port and how big/ long should it be ? pictures would be awesome! i have a drawing sorta.