the top of my box inside is 3.5 inches wide and 60.5 long.Thats where i want my port can i just leave the top open and cover the sides just leave my port a specific size. Also would i have to build walls back down for my port or just cut a slot?2 mtx t81044a in a box im trying to make about 2.66 cubic ft. also should i subtract .14 cubic ft cause the vent?