I am getting ready to build a box for a btl n2 18 and I found these cut sheets on the forums Fi links to from their home page. http://www.soundsolutionsaudio.com/forum/topic/44518-btl-box-cut-sheets/ The dimensions just so happen to be perfect to fit in my escape in the area I have left. I have 3 questions. Is this a decent box design? Is there any problem laying the box on its back to fire sub up port up? ( too close to back hatch I think to fire back ) And it shows the hole the sub goes in as 16 3/4 diameter. Is that right for an 18 inch btl? I figured it would be close to 18 inches?