Tops for the Liners
I use 1/2" foam for the top of the liners. If you want a more pronounced top you might go to 1" thick, but use the same methods for 'flicking' the foam out as we have already done.
I made a template out of paper to know the shape and size I wanted. Because the liners are going to be covered by other scenery (and dark) I'm only going back so far. If you are able to see through the tunnel, I'd recommend to extend this the whole length.