I have this tunnel structure from someone here on the forum, so it's not a collection item. When I move the green box the slightest bit on the floor plan, it jumps to the top of the tunnel. Every time I move it I have to set "height above ground" to 0 again. It seems the tunnel's bottom does not serve as a proper ground. Drawing a room around it did not help. Any way to fix this?
You should specify "dropOnTopElevation" (value 0 or -1). Create library of your model and edit PluginFurnitureCatalog.properties or edit Home.xml (part of sh3d file). Library(sh3f) and project (sh3d) are zip archives - use any zip tool to extract and add files.