@MV, what I did originally was cut some 1/4" square wooden pillars, put 2-part epoxy on them and glued them in strategic places onto the sheet metal. I then countersunk 1/8" rare earth magnets into the tops of the wooden pillars. The 2nd and third layers of metal stuck nicely to them.
However, something dropped (I think I was showing a friend how the magnets all held upside down and, well, turns out I needed bigger magnets...) and the pillars broke off.
I COULD take some type of broadheaded nail, drill a hole and run the nail from underneath, giving it a stronger foundation. I'd need to greenstuff magnets onto that to make it stay reliably. At that point I was groaning from the silly amount of extra work I was putting into it and went out and bought a metal tool box

My tray looks identical to AoC's except my handles are blue and not green. The killer toolbox I got for $50 is 20 freaking pounds but is built like a tank. As soon as I finish basing all my tanks with my Shogun Miniatures custom magnetic bases, I might actually use the damn thing LOL!