heavy steel EA (even angle) - 2 of - running across two bearers in floor under safe
12mm coach bolts up through the 4 holes in the floor of the safe, nuts and washers only accessible from inside safe
Easier to get the distance right between two points with 2 pieces of steel rather than a flat plate with 4 holes
steel is about 10mm thick from memory
That's how I've done it
I'd recommend inside the house rather than the garage
Edit- If I did it again I'd use heavy channel as it is harder to poke a grinder in there to cut the bolt heads off from underneath
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