my buddy is thinking of buying my 8 inch solo baric subs. he wants me to build him a ported box. he doesnt have enough room for the ports on the face of the box. so i thought of making one port for the 3 subs. the kicker specs for the dims are 2 x 9.5 x 21.25. how do i make one port for 3 woofers? basically what do i do to figure out the new port? do i add the numbers together or what?