Just a thought here...
How about using the frame structure from the existing dash for the mounting points. But instead of using styrofoam or floral foam to use as a form to be removed later, could it be done (while still in the car) by covering the ducts, wires, etc. with saranwrap, and apply a layer of a (for support) fine wire mesh (such as window screen). Now you could spray a foam such as that aerosol home window sealing insulation onto the wire mesh and get to carving and shaping with some files, rasps, and sand paper. Then the plug that you have just created could be covered with some fiberglass or carbon fiber. Now instead of a thin dash cover, you would have a solid and removeable dash while maintaining heater ducts and electrics in thier original positions.