sub box recommendation

hey everybody   i have a 2006 mustang gt   i just bought a soundstorm 5000 watt mono amp and a kicker 15 inch l7    i listen to music with a lot of boomy bass that hits real low frequencies   i dont know the dimensions of my trunk but i know that it is very small   what kind of box should i get and what size?   the amp is rated at 3000 watts x 1 at 2 ohms but i find that hard to believe but i think it is enough to push the l7   does the amp have enough true power to push a kicker solo x 12   if so then i will get one   if i do get a solo x12 what kind of box and what size for my mustang    thanx