Hahaha yeah general convention used to be just throw them all in a bin like nonrechargeable batteries. Train explosion in Houston a while back showed why that was a bad idea. That said if they’re all completely discharged, the hazard is much less because less stored energy AND the cathode is more stable (gives off less oxygen during decomposition). The former reduces internal short hazards/heat; the latter reduces energy and effluent release if they go off.

The fireproof container isn’t a bad idea, but be careful of making a bomb. You want it to leak before burst just due to the large volume of effluent produced. Best practices are to reduce how much you store, keep long term batteries at ~25% SOC (less for disposal), and storing away from other flammables including other batteries.