Here are a few pictures of my tunnel and mountain base.
I over purchased too much of the risers so I used them for the Face of the rock wall.
Hot glued them together and I put a few threaded rods in the lower pieces for strength.
The back wall of the tunnel I used sheets of 1 inch Styrofoam glued together to make the walls nice and strong.
My next move is to design the mountain on top of the base and then give everything a coat of Plaster Cloth.
After that the rocks will come.
Keep you posted.