i built a box for a 15" bl but like an idiot forgot about port area and i only have about 29 sq in. i'm building a new box today how do these specs look correct any if wrong. 20" wide 25" tall 25" deep port is 3.5 tall 18.5 wide about 37" long . box is 4.2 cuft tuned to 30 hz. speaker up port across the bottom facing back. looks like about 64 sqin of port.