Originally Posted by
saddlesores
depending on the size of your derailleur, a 1.25 or 1.5-liter plastic pop bottle can easily be surgically sliced so's you can slip it over the component in question and ducky tape or zippy-tie it securely to the chainstay for a bit more protection.
When I bought my road bike, it came in the factory carton packed at the factory. Derailleur was attached to the frame (steel, no separate hanger), there was a plastic fitting attached at the skewer that stuck out further than the derailler did, so an impact from the side on the box would hit the plastic spacer instead of the derailleur.
Not the best photo, this was cropped from a much larger photo, but you get the idea. Photo was from when the bike was pulled out of the carton, but packing materials not yet removed.
If I was going to transport a bike on a plane, if for some reason I decided not to remove the derailleur from the hanger, I would see if any bike shops had any of these that they had not yet discarded.