__outside object

The '__outside' is defined as the outside perimeter of the entire groundpoly for your airport. When making the groundpoly, that would be the area that's colored as black (transparent) so that your airport blends into the scenery. You should have at least that parameter along with '__runway'. You can also include '__apron' if you plan on making a textured apron instead of just the standard ground textures.

You can, in theory, just place a runway object onto your airport, use a reference image and place objects onto it but the result will only yield models siting on top of whatever scenery images that you use in-game.