how do i get the air space for this box

Pretty close... I'm getting 3.11 cubic feet per side, outside dimension, and GROSS. IF you measure internal dimensions, subtract the bracing, and subtract woofer displacement, I think you'll be pretty close.It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

You can figure it one of two ways. 19*17*20=6460 cubic inches MINUS 9*6*40=2160 cubic inches 6460-2160=4300/1728=2.48 cubic feet. (I must have done something wrong on the first calculations...) The other way: 10*17*20=3400 cubic inches PLUS 9*11*20=1980 cubic inches 3400+1980=5380/1728=3.11 cubic feet. (Apparently, that's the way I did it... ) Basically, you just figure it as two volumes, and then add them together. If you figure the INTERNAL volume, it'll be the same, either way you go. I figured external volume, as stated earlier.It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

The INTERNAL volume of your box assuming you are using 3/4 inch material and a 3/4 inch divider is 4.69 Cu Ft.   This is the actual box volume I did not subtract the volume of your woofers.     Just so you will know how to figure your box volumes on your own. The formula for a simple square or rectangle box is as follows. Since you are wanting to know the INTERNAL volume of your box you will need to use the Internal dimensions of your box.    Length X Width   X Height (in inches) = Cubic Inches. Then you will need to divide that number by 1728. That will give you Cubic Footage.     The box you have is a little bit trickier with the offset at the bottom.