I was planing to make a box for my BL 15... but i have a few ideas.. whats the diffrents from the sub facing toward the rear of the vehical? Vs. The sub faced up ward toward the trunk lid? and what is a good tuneing for the sub? 35hz? 32hz? or 30hz? i have a 1500 rms amp... and i listen to rap and metal.. rap most of the time...

What kind of vehicle is this going into? Stick to Fi's enclosure specs, 12-16 inches of port area per Cubic Foot. So, an enclosure that is 3-3.5 cubes after displacement of the subwoofer and port should work fine. I am tuned to 32hz (18" BTL) and it sounds pretty good, port facing rear and subwoofer firing up 9to the roof in a yukon xl.

3-3.5ft^3 with a 30-32hz tuning and 45-55in^2 of port area. The port is going to need to be long, but you need that large port area with such a capable woofer.