Ken:
Think there may be an easier way to make the scene movable rather then using foamcore. What I have done is apply the modeling fiber to construction foam over an adhesive drywall mesh. Once dry you can lift off the fiber sheet by using a putty knife or spatula. It lifts off very well once you get it started, and don't worry about whether the drywall tape stays attached. You can cut the sheet with heavy scissors. Wet the surface you are going to apply it to on layout and press it down.. Maybe use a touch of dilute glue around edges.
I did a plowed field that way and recently have done a number of dirt roads.
Here is photo link
The largest sheet I have done is 2 by 3 ft
In this case a razor knife was used to cut dirt road out of a demo I did at show. I put it in inventory for future use. 
Thanks for using and trying product
Bill D
