I always thought it would be a good idea to get a huge slab of 1/4" steel. Like, 4'X6' and bolt the safe to it. It would distribute the weight and, it would be very, very difficult to move. Like, wouldn't come back out of the closet bolted together, couldn't use a dolly etc. It would be alot heavier and, damn near unmanageable.
Or, if the weight it too much, two 1'X6' slabs of steel.