I have added a layer of plaster to the mountain ridge.

I used a product that consisted of a gauze like material with dry plaster embedded in it. You just cut a piece off the roll of plaster wrap and dip it in water. Then you apply it to the area you want to cover. You can smooth it out with you fingers. After it dries you will have a solid surface to apply scenery to. You can add additional layers for more strength. This was a little messy, so I was not able to take pictures during this stage.

I first applied some odd shaped pieces to frame in the tunnel entrance. I then applied rectangular pieces along the base of the ridge. I continued adding pieces working from the bottom to the top. I tried to overlap the pieces by an inch where possible. I also had to line up pieces where there was cardboard to support them. I let this dry for a few days. I will need to fill in the areas where the plaster wrap sagged.