When I worked in the steel fabrication shops in the oil and gas fields we often made steel boxes to store explosives in. They had a simple method of making the lock so one could not break the lock and get in.
I then made my gun boxes the same way but made it extremely tough to get into them even if you have a key. The only way to break in is to use a cutting torch or grinder. And to make that even tougher I welded brackets on the inside of the boxes near the locks which I stick cans of GOEX black powder with the lids off. Any touch with a torch or grinder and BOOM!
I rarely open that gun box as it is so hard to get into!
I also do some welding.
Would love to see a photo of the secure fix for the locks.