This might be oversimplifying it, but just putting a cable through the trigger guard will slow the average smash and grab guy down. I've personally never tried to break the trigger guard on a gun that had a cable through it, but it definitely will make the smash and grab guy stop and think. If the bad guy has a cable cutter, it does not matter if the gun is in a small safe, attached with a cable, or not. I guess this is one of those, better than nothing suggestions. I think that if the bad guy tried hard enough he might damage the gun, but my objective is to keep the gun off the street. I will buy a new frame it that is the only damage and I've "saved just one bad guy from possibly hurting himself with my gun." /s/
I have a pretty hefty cable with a combo lock built in, that was sold as a bike lock. I have used it in a number of situations when, it was "that" or nothing. I carry scuba tanks and lead to one of my dive locations on a hand truck. I hook the hand truck up to a tree. I can only speculate that a lot of bad guys have kept walking in order to find an easier target. Yes, cutting the tree down would make it easy to steal my hand truck, but I hope you understand my rational. Have you watched a bad guy pick up a bike, that is locked to a no parking sign, and simply lift the chain over the top of the sign. Sometimes bad guys are smarter than the average city dweller.
I guess if you really wanted to you could drop the mag and pass the cable through the mag well and out the ejection port. Short of cutting the cable that gun is staying put. Personally I don't recommend loading and unloading in the car or truck on a normal routine basis. I guess putting a cable though the trigger guard of a loaded gun has its own issues.
But hey, guns are inherently dangerous right. I'm just thinking out loud.