This is the best solution for protecting your rear derailleur and derailleur hanger. Simply loosen the quick release, insert the Guard, and close the quick release. Made of a very tough plastic, it will protect your bike from strong side impacts. Likewise, it is great at keeping grease from the chain off any surface.
Using the Rear Derailleur Guard in a bike case or box is easy as well. When the wheel is not in the frame simply use a chain-tensioning quick release or spacer and the Guard will stay in place.
* – not designed to work with rear through-axles