Just an intersting thing that I've mentioned before regarding gun safes, and many manufacturers have safes that will fail this test.
To do this test, open your safe door about half way. Assuming you're right handed, raise your right knee and place it about half way up the door near the opening edge. With your right hand, grab the top corner at the opening edge. Push with your knee, and pull with your hand, and see what the door does. Put as much effort into it as you can, as you won't hurt the safe.
What you may see if you look closely is the door flexing back and forth. Some will be a lot more obvious than others.
The moral of the story: If you can bend the door by hand, it is probably not as burglary resistant as the manufacturer led you to believe.
The Fort Knox Defender does have a 1/4" door. It should pass this test with flying colors.