I am building a slot port box for a jl w1 10" sub. How big should I make the slot port? I made a box last week that was 18" by 14" by 13". The vent was 14" tall, 1 and a half inches wide and about 20" long. I could hear the air coming out of the vent, and it didnt sound too great. What should I do to the box I am making now?