Great and informative thread. When I travel with a handgun (if I'm traveling around the USA I WILL have a handgun with me) I use a hard sided suitecase which I modified by installing (bolting) an "eye bolt" purchased at a hardware store. The inside of the "eye" is, naturally, inside the suitcase body. The little bit of threaded shank that sticks outside the body of the case I put a plastic cover on (ACE Hardware had all the stuff I needed for this). I had a plastic coated cable permanently attached to to the eye of the bolt. There is another loop around the other end of the plastic cable, which I attach, via the same lock used to secure my Pelican case which contains the handgun.
Could you steal my little handgun case from my luggage? Sure. But you'll need to have a pretty heavy bolt cutter or cable cutter in your back pocket in order to do so.
To make everyone's life simpler I'll get the case out of the basement and photo the set-up for this thread.