Converting a 4x12 cab to a 2x15 cab...
 

I'm planning to convert this guitar 4x12 to a 2x15 bass cab. I'm planning to have the 15s offset diagonally. The dimensions are H 30", W 30", D 15". My question is what would be the best way to port the cab? And how deep should the port be?
I realize that it isn't really an design to work with for most bass players but its what I have to work with. I already have 2 4x12s and 1 4x15 in my rig and I would like to add a 2x15.
Any help would be appreciated, Thanks!

Too small for a 215 bass cab would be my bet. 115 might work.