You can actually use any landclass to create a background for an airfield, the trick is just which flatten to use.
Most people (including me, normally) just slap down an AB_Flatten_ExcludeAutogen_MaskClassMap and you end up with a green (brown in desert areas, white in snow areas) shape. As Bone says, though, aerodromes (covers all bases

) usually do stand out quite well from the background - you just need to pan across an aerial view of East Anglia to see all the disused ones out there and how easily you can pick them out.
You can actually use any combination of those, placing them over a landclass polygon, and you'll end up with a background that will blend in much better. It works better on tundra or outback landclasses than a city or forest, obviously, but it's not difficult to lay down your own if you can't use what's already there.
What I was referring to before, which it seems didn't come across very well, is that in FS, aerodromes tend to be bounded by a very specific, moulded to the shape of the runways, cut out. Things are rarely that well defined in the real world although a lot of that is to do with the simplicity inherent in having the sheer number of airports that FS includes - I very much doubt they created all those flattens by hand!
What I've tried to do when I've done a scenery is to follow the contours or a defined boundary with the flatten or exclude so that at least you don't get the perfectly symmetrical (sp?) shape around the runways. I think it looks more natural, but it may not be so much in places where the room and willpower exists to actually put an airfield boundary exactly where you want it.
Does that make more sense?