Fella's;
Sawdust beat me to it. The very thing I was going to suggest. Square steel tubing on the perimeter & a bolt down thru the interior.
More work, more money, is a concrete pad several inches high. the container rests on say four small sections of square tube & the bolt is left either hanging from the interior down, or placed on the base & projects up into the safe. Then concrete is poured, left to set & the forms removed. If you put the head of the bolt down, you can then place the nut inside the safe. If you need to move & take the safe with you, it's possible. If you think that's what you want to do, wrap the bottom & about a foot up the sides of the container in heavy plastic visquine. And be very careful on the pour.
900F