Couldn't the thief simply pull up on your gun case and cut the cable through the gap created? There's no way you're going to get the cable tight enough by hand

Couldn't the thief simply pull up on your gun case and cut the cable through the gap created? There's no way you're going to get the cable tight enough by hand than a thief can't move the case a bit (safes generally use bolts, which don't have this problem). Maybe if it's secured to the back wall it will be awkward enough to maneuver the cutters that the thief gives up?