Thanks for the encouragement, Howard, and thanks again for providing the project details.
I got the liner fitted over the weekend, but it was a bear:
File comment: Tunnel liner
HL project b.jpg [ 152.25 KiB | Viewed 11047 times ]
I was going to use cereal box cardboard for the liner, but couldn't find any that was long enough (possibly the result of me lengthening the tunnel by 4-5 inches). Reverting to corrugated, I had difficulty getting it to take a graceful arch form. I used an old shipping tube as a sort of form, but the cardboard had a tendency to bend incrementally rather than in a nice curve. The ends look okay because they are glued to the form of the portals, but at the middle the ceiling is a bit notchy.
The other problem was I neglected to take into consideration the tilt at the top of each portal caused by the pitch they were cut on. When I glued in the liner at one end, a noticeable down-hill angle resulted. I had to force-fit the opposite end using a quite a bit of Titebond and a half-dozed C-clamps. The combination of the force-fit and the poor arching at the center resulted in a very slight hump. Twenty-four hours to dry, then I trimmed off the excess corrugated.
Presumably, the next step is to manufacture a couple of supports that sit over and atop the liner. These will hold the base of the structure that goes above. I'm thinking of using one-inch foam for these, but if that isn't strong enough, I'll use some scraps of one-by lumber. I've left the liner unattached to the side rails in case I need to do a bit of fiddling to get things to fit. When this bit is done, I'll glue the liner to the side rails.
Not to get too far ahead of myself, but I do have one question. How do you guys separate corrugated to get at the rippled bit that you used for the building's roof and some of the trim? Or is that material from somewhere else?