Boron nitride powder. Cheaper than diamond. Cheapest way is to get sand (Aluminium oxide) and mix it with a binder. Wears down anything, same as sandpaper. Layer that over and around your hardened steel ball bearings or TIN bits, and no-one is going to drill more than a small amount of it. Pour it onto the thick-ish steel plate you have already keyed with small pits, etc. for grip. Treadplate is quite nice for this. You might want to harden it a bit, though, as it could be cold-chiselled if thin.
Use something like glass or epoxy resin as the binder, and even someone with a gas torch is going to have real issues, since the stuff burns so bad with nasty fumes. Same with a disc cutter. Plus the inside of the plate is harder than the wheel!
Another trick is to put a lot of loose threads of something tough, like Kevlar, in the mix, so the threads tangle and stop any diamond discs or rotary tools, since they will pick up the threads and tangle.
Make a lock that is quite pick resistant, then mod the key so it is longer than normal, or round a corner, so that an inch of this plate is between your vital spots and the possible driller.
On the inside use a glass plate that will shatter to release additional locking pins, if you must. Have a good relocking system, make sure the safe handles are weak enough that they can't be used to pull the door off or the safe out the wall.
Put lots of rebar around the frame so it is set into the wall solidly, and to stop anyone hammering the safe loose.
Get a web-cam set up to monitor the area, with motion sensor. Watch from afar as the attackers use a 32 tonne JCB (digger) to rip your safe out and take it with them on a flatbed truck, after you get ram-raided by a lorry. Sell the footage to a reality TV show and buy another safe.
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