Cannot create wedge box

There needs to be a second option for Height.  Front of box height and Back of box Height.  I have two boxes I am trying to create and cannot because there needs to be a second height option for a "wedge" box.  Example

Front of box height 20

Width of box 32

back of box height 9

Bottom of box depth 30

I need the box to look like a piece of cheese with the port in the front for one of the boxes.  I have tried to recreate this with the "trapezoid" functions but nothing allows me to make a flat back of the box.  

Sounds like you need a double wedge. Try this link Woofer BoxWizard | Rockford Fosgate ®