After playing around with the map editors a while, I've a couple of simple offerings that will hopefully be of some use:
Orchard Much like the one in the original XCOM game. Size (10x10).
Submap Data (Preferred Folder Name: "SMOrchard")
Farm Storage Shed A single storey concrete and corrugated iron storehouse for fertilizer or diesel. Size (10x10).
Submap Data (Preferred Folder Name: "SMFarmstorage")
Grass Field A basic, large grass field for use with constructing larger farm maps. Size (20x20).
Submap Data (Preferred Folder Name: "SMGrassfield20x20")