box dimensions

Here's a starting point:  figure the square inches for a 4" round port opening and convert it to the rectangle of your choice.  Use pi R^2 to calculate the area of a circle, so the dimensions you have to work with is a 12.56 square inch opening that is 13.5 inches long.  Now all you have to do is decide what size slot opening you want that will come to 12.56 in^2, and leave the length as it is.  So decide what the longest side of the slot should be....let's say it's 12".  12.56 / 12 = 1.047, just a hair on the big side of one inch in width.  If you want wider width, use less slot length such as 6".  As long as you convert the circle to an equivalent square inch opening, the length of the port will remain the same.

Here's a link to an MTX box that has dual chambers and the slots share the common wall:  http://www.mtxaudio.com/caraudio/products/enclosures/sledgeHammer8500.cfm

To find what size to make the box so that each chamber is 1.3 ft^3, here are the basics:

12 x 12 x 12 = 1728 cubic inches = 1 cubic foot

1.3 cubic feet * 1728 = 2246.4 cubic inches.

Find three measurements (in inches) that when multiplied will arrive at 2246.  ( ex.:  15 x 15 x 10 = 2250).  Add volume for port structure and driver displacement, using these guidelines, so that the overall dimensions will net 1.3 ft^3 in each chamber after deducting the driver and port structure.

Then make the chambers mirror images of each other.