Inserting a panel in a truncated pyramid?

Hi Guys,

Is there a simple way to insert a panel in a truncated pyramid shaped enclosure? What I have done so far is : started with a sketch made a square. then extruded it under a 45 degrees angle. Now I have the shape I want and I used the "shell" option and later I split it up in 6 parts. So far so good. Now I want to insert a panel in the pyramid under an angle of

Thanks for suggestions

Edmund