For underneath, in a perfect world, I'd use the Grace Peel-n-Stick Ice-&-Water flashing, stuck to the bottom of the safe. But, that could be complicated w/o a forklift.
So, the next option would be anti-fatigue floor mat, cut to match the safe dimensions as near as possible. In a better world, I'd have the mat come out as far in front as the door opens. But, many of up live with Significant Others who would be more concerned about the 80% of the time the safe is closed, and how the mat might not match the "design" of the rest of the bedroom.
Another, fallback item would be to get the neoprene lock-together "play" mats, as they can be trimmed neatly to suit the safe.
Now, if there are rust stains on the carpet anyway, let me advance an idea. Get a carpet person out, and trim the carpet out from under the safe location first. This will give a number of benefits. Like being able to more-neatly drill the slab. But, mostly, if the Design Committee meets and she tells you the bedroom carpet needs replacing, you will have dodged a serious bullet for labor and expense.
I'd not use plywood unless it were marine grade waterproof, not in as damp a climate as Houston. That carpet wicked moisture up under the previous safe , which is where the rust came from. Anything but waterproof wood can act like a sponge.
