For Down Ports I sometimes use google earth images, plus the supporting maps/layouts of existing large airports. I've used Tampa International in a recent adventure. Landing starships will still come in on a glide path, thus a run-way layout. Runways don't need to be as long of course, but IMTU most down ports don't simply have a bunch of landing pads you just simply land in. You land on a designated landing strip, then are guided to your proper gate. Smaller downports tend to have the kind of layout as depicted in some episodes of Firefly, like the Eaves downport, or the old standby, Mos Eisley.