change port from round to slotted

what size of a slotted port do I need to make if it is supposed to be 3" round  14.36" long and I need two

Round to Square Port Conversions:

Converting round ports into square/rectangular ports in speaker enclosures is just a matter of math. For example: If a subwoofer calls for a 6" port that is 13" long, take ½ of the port size, in this case it would be 3. Multiple this number by itself, so 3x3=9. Now multiply the 9 x PI (3.14) and this is the total area of the square port opening or 28.26 square inches of opening. (If you notice you have just use the area of a circle formula (Pi x r (squared).) This size opening can be built using any geometry, such as 14.13 X 2 inches, 9.42 x 3 inches, etc.

If you speaker calls for more than one port, multiply the number of ports by the number you calculated to get the combined calculated port area needed. As for how long the square port should be, the length stays the same as the round port. The reason is because the length of the port determines the tuning of the port and the length is not effected by the conversion from round to square ports.

Exactly.  Consider the volume of the port to be a solid object when you're calculating box size.

Scott Gardner