Lol, not model, but path texture.
You have to make the texture for your pathing. (in this case image dimensions don't have to be a power of 2).
Create an image in dimensions AxB (for example 6x6 if you want your pathing to be 6x6).
Now you have to color every pixel of image with color representing specific pathing:
light purple - RGB(255,0,255) (unbuildable, unpathable, flyable).
black - RBG(0,0,0) (default)
white - RGB(255,255,255) (unbuildable, unpathable, unflyable)
blue - RGB(0,0,255) (unbuildable, pathable, flyable)
You want a path for a building, so you will be interested in light purple (RGB(255,0,255)).
Color every pixel with this color. If you want to have some fields pathable, leave them black (RGB(0,0,0)). 1 pixel = 1/4 of medium grid.
You can also make the blue frame to give your building waygate.
Save your image as TGA bitmap, import into map, and set as path texture for your factory (path doesn't matter).
If I missed something or you didn't understand something let me know.
Examples:
Unflyable 4x4
City Arch