At a minimum adding 200 pounds to their weight should be easy enough, even with steel. If they use lead, then close to 1000 lbs. should be doable. Obviously the weights are added after they’re installed.
Anything wrong with bolting them to the floor?
Breaking Bad had a hilarious episode about meth-heads who stole an ATM and could not figure out how to get it open after they took it home.
The weights are an excellent plan , dye packs and AirPod type trackers. Combine all three and thefts will drop. Oh and a skunk juice bomb designed to go off with the dye packs, just to piss them off.