This page will guide you through the process of choosing the correct sub and layout of your enclosure. DO NOT buy the sub first instead do a rough side profile drawing to determine mounting height and volume. Subs that fire down are well protected but yeild the least amount of volume to the enclosure. Volume is lost raising the sub off the floor in most cases this will be a SEALED setup. Subs that fire forward maximize the underseat volume better suited for SEALED and Ported setups. The drive line tunnel and seat brackets are another variable to consider when making your choice. MOST stock underseat enclosures without a driveline tunnel or brackets will provide 1.5 ft^3 net.
In this profile we will use a SA-8 you will need a minimum front height of 10" and a rear height of 7.5" with a 13" depth. In most trucks this will not work without doing a 1.5" seat lift. This setup maximizes the most volume available. This configuration is better suited to displace a port. The tuned frequency can also be raised to 38hz to save more volume displacing a shorter port length.
In this configuration we are using a low profile JL Audio 10TW3 sub. Most stock seats have 6" rear height, 8.5 " front height. In the this drawing the bottom depth is kicked out to 14.5". The kickout allows the sub to move forward increasing mounting depth and volume. The bottom mounted clearance to the floor is 1.28" for excursion(2" overall).