Some mid-week progress:
Cut a 1/2 inch piece of plywood 4.5 x 5.5 for the base. Added thin strip, 3/4 inch high, to the sides to cover the plywood and form the cavity for the ground work. This was covered with painters tape prior to adding the ground work and future painting.
I'm old school and stick with what I learned from Shep Paine's book and use celluclay for the ground work. I also add white glue to the mix to help stick to the base.
Spread the celluclay onto the base and smoothed out with wet fingers. Pressed the first layer of rock into the groundwork and added worn tire ruts, which I also added tire treads using the kits tires.
This will sit for a few days to dry.