Last night I spent a little time looking through E. L. Moore's EVRR photos for a good picture of the tunnel portal that appears on the level below the stone one. These 2 snippets are all I could find. It's some sort of wooden crib and box-like structure. Clearances are tight in that area of the layout, so some careful measuring will be called for before I start to build. Also, there's a switch just inside the portal, so that'll complicate things.