Cheetah: Reference your question, "what are the old wooden tunnel portals made of"? Answer--- Wood. Or if you are talking about the model ones, cast plaster or wood. The colors of the wood will depend on its age. New creosoted timbers will be almost black. aging turns bare wood to a greyish color. You may find several shades on one piece.
I noticed an article in a magazine awhile back about a particular kind of party balloon that has a good simulated water surface.