I'm new to all of this but want to learn so bear with me. I'm going to build a fiberglass box. Half MDF(bottom) and fiberglass(top). I bought a Alpine SWR-1222D Sub and want to mount it with the magnet fasing out. It say's that the recommendations for a sealed box(which I want) to be 0.071 -1.0 cubic feet and for reverse mount(magnet out) which I want. It say's added volume 0.085 cubic feet. What is ADDED VOLUME mean? My other question is it say's the external dimensions of the box should be 13"x13"x12.5". Does it have to be that dimension since i'm reversing the magnet? The box I want to build is (external dimensions of MDF) are bottom 18" wight 14" front side 7" back side 5" and the top will be fiberglass with the height being 13" from bottom of MDF to the top of the Fiberglass. Is this okay or do I need to make the box lenght shorter and if so what would you recommend?