circle to slot port conversion?

I have a speaker box design that sounds great and has a circular port, but I am about to rebuild this box so it will fit into my new car and would rather have the port be a slot port...  I do not want to redesign the entire box, I just want to make this small change.

Does anyone know of a formula to convert between the two types of ports?

Thanks, e_lectro

These are some personal notes to my self that I wrote many years ago
 
NOTES ON CALCULATING PORTS
SURFACE AREA OF PORT
To get the square inches of a ports surface area you multiply ¢³= 3.14 times circumference of the port. IE (4¡±)
if you have multiple ports you then multiply the results by the number of ports that you have. So if you have (2) 4¡± ports then you calculate as follows
3.14 X 4 = 12.56  12.56 X 2 = 25.133   25.133 Is the surface area of the two ports now combined
four is the size of port 12.56 is the surface area of one port the X 2 means that you have 2 ports and 25.133 is the total volume of both ports.
HOW TO CONVERT A CIRCULAR PORT TO A SQUARE PORT
If you want to use square ports, use a port whose height multiplied by its width equals the 'port area the port length will be the same as the round port.
Let¡¯s say you want to convert a 4¡± port which is 14.5 in long to a square port. Multiply 3.14 X 4 = 12.56 now calculate what times what would = 12.56 or 12.5 2 X 6.25 =12.56 so you have a port that is 2 X 6 ¨ù X 14.5 long 
Or if you have 25 sq/in of surface area and a length of 14.5  you calculate:  25 % by 2 =12.5 so you have 2 X 12.5 X 14.5 long.
A good site with a port calculator is https://www.eatel.net/~amptech/syndicate/spboxnew2syndicate.htm