Welcome aboard, Gomezy3k.
Yep, the best place to get answer on your dio is the Diorama Forum.
I agree with making the mountain hollow. I have worked on several train layouts and that is the best way to go.
I cut a few pieces of wood to support a screen. I then stuff crumpled paper under the screen to give some support. You can then dip paper towels into plaster of paris (or similar) and lay over the screen. It's a good idea to color the plaster with brown or green food coloring. This way, if it does chip, it doesn't show as a bright white spot. If you can, try to use the paper towels you find at a hrdware store. They tend to be a bit rougher and more paper-like (not like soft paper towels)
Once dry, you can add more plaster to give it some thickness and add grass, scenery and such.