Would a 3/4" piece of plywood be thick enough?
Are you mounting it between the safe and concrete? Or are you using it to help support it on a std wood floor? If you have a moist concrete floor use a sill gasket material or sandwich some plastic between two pieces of plywood. Is the area prone to flooding with a little water? By a little I mean 1 inch or less. If you want to displace the load on a wood floor how much does your safe weigh and what is the footprint on the floor space? And what floor of the house?