Round Port To Slot

[candyman74]

ok i have a question that im needing assistance on. im making a box for a 15 inch power acoustik mofo, i know im just tying to make some money on the box. it calls for a box size of 3.4 cu ft tuned to 40hz with 2 4" ports that are 8.24" long. ok im wanting to convert the round ports to slot. i know the formula is 3.14 x radius2 so that work out to be 3.14 x 16 = 50.24 so then i need two so that would = 100.48. now to me that seems like a lot for one 15 is that right? the box is only 17 inches high making the port 15.5 inches high and that would make the width of the port 6.48 that dont sound right to me please help. to help the box is 28x17x17