you might also try Woodland Scenic puts out sheets in different sizes that will fill your needs, then purchase some HydrocalScultamold to cover your work.
https://www.allartsupplies.com/item.php?articleId=567&gclid=CP2v3aOll54CFSZdagodrUrblw
Woodland Scenic
http://www.woodlandscenics.com/show/category/SubTerrainSystem
https://www.allartsupplies.com/item.php?articleId=567&gclid=CP2v3aOll54CFSZdagodrUrblw
Woodland Scenic
http://www.woodlandscenics.com/show/category/SubTerrainSystem