Updated 5/22/2012:
Got a chance to work on the access hatch last night, finally attaching the bottom layer to the rest of the structure.
In this photo, the structure is upside down on my workbench. You can see the hole for the power wiring leads and the terminal strip to which they will attach.
In the second photo, you can see the wide or slag pit end of the access hatch. "Blue" or "Pink" foam will be used as a filler to stabilize the "sandwich" and carve out the slopes of the pit.
Next steps include playing with the fit into the rest of the layout, applying a coat of paint, and installing Vinylbed, track, and wiring. At some point soon, I will take a shot at carving the filler.
George