Slot port. Using cabinet walls. How to correct? Help!

I am making a subwoofer with a slot vent. It will be using the bottom and sides as part of the vent. I know that a speaker box calculator will show how long a vent should be if it's a regular vent, but with 3 out of 4 of the vents walls continuing past the port, It must be tuned lower and there must be a way to correct it.
I made a pic to demonstrate what I mean.

[IMGDEAD]http://home.comcast.net/~tarnationsauce2/port.gif[/IMGDEAD]



The finished product will look something kind of like this:



[IMGDEAD]https://imagehost.vendio.com/bin/imageserver.x/00000000/progearwarehouse/GSS18_fullsize.jpg[/IMGDEAD]