I've been following this thread and it is impressive how quickly this is coming together, given all the details you're doing!
With deep winter quickly descending, you've gotten me motivated, and I've been contemplating a micro crawl course to go on top of my unused slot track. The track will require lots of elbow grease to get it back to tip top shape, so that's a project for a distant future. So, I've been thinking about making a modular, lightweight system that will just sits on top of the track layout.
What I'm thinking is to use 1" rigid foam insulation panels, cut into something like 2'x3' sections. Hot glue ground and rock structures using cardboard and/or chicken wires onto the panels. Then paper mache for the final finish.
Doing a little research on paper mache, I came across this video, which I thought is brilliant, so figured I share it.