If you REALLY want to get medieval, bolt the safe to the floor, about 8" out from the wall. Plug the unused bolt holes. Surround the safe with plywood forms, standing the sides out about 6", and making the height about 6" taller than the safe. Use a silicone sealer to hold the front plywood panel flush with the door of the safe.
Construct a cage with rebar that surrounds the safe, leaving the front open. You can go cheap and use chain-link fence cut to size, or omit it altogether.
Get about 15 bags of Sakrete concrete mix, a large plastic kiddie pool, some 5 gallon plastic pails, gloves and a case of beer. Friends would be helpful at this point.
Mix the concrete and use pails to fill the form from the top. Level with a trowel when you reach the top of the form.
Suggested modifications:
1. Drill two small holes in the top of the safe to accomodate fittings and some 1/4" copper tubing. Run the tubing straig out the top, about 4" above the anticipated concrete level. Drill a small hole in the bottom side of the safe and thread a heavy duty extension cord through. Seal hole with silicone. This will allow some small amount of air circulation and you can run a goldenrod to eliminate moisture.
2. Affix 2 large hardened eyebolts at the sides of the safe before you pour. Use them to attach a "lock bar" across the front for extra protection. Use two sets, top and bottom.
This is all kind of like polishing a turd, but it's cheaper than buying a better safe.