D'Addario Planet Waves PW-GD-01 Guitar Dock

This guitar dock from D'Addario is a great little item that allows you to dock your guitar just about anywhere to anything.
Simply adjust the clamp on the back to secure it to the edge of any ledge/surface, and the front will hold onto your guitar gently and securely.  
it can handle guitar neck widths up to 5.4".

It's portable, lightweight, and it even rotates to allow mounting at any angle. 
You can store it right in your gig bag.