Don Fiesta

Hello, im from Belgium so sorry for my bad english, i hope you will understand me. im planning on buying 2 Sundown SA 12's and im going to run them on a Massive Audio N3 amp. and i need your help and thoughts, i know a little of how to calculate a box for the subs bud im not not that good in it. 1, i drive a Ford Fiesta mk6.5 and my car runs on 14.3/14.4V 2, Does the 12's going to be too big for my car? i want A LOT OF BASS, i love the very lows like decaf songs etc... 3, i will do the big 3 upgrade to my car bud i would love to keep the stock alt, is that possible? 4, will someone plz design a box for me for the 2 12's? i was thinking about a 30Hz tuning, subs facing up, and the port in the middle of the subs (also facing up). or would it be better to face the port to the back? my trunk dimension i can use are: 45cm height, 97cm wide, 45cm deep. I really hope you will help me out with this, i dont know that much of subs/amps/box making, i ony installed some entry lvl sub/amps an made 1 box for a kicker L7 12" so i dont have that much experiance. and if you agree to help me out with this, will you plz design a box in centimeters / millimeters with 18mm MDF? because here in belgium we only got 18 or 21 millimeter MDF. (not 19mm like 3/4inch and with torres calculator you only can use 19mm MDF) You don't know how much i would appreciate it if you would help me... Plz let me know something... grtz