FireKing International manufactures a 1-hour fire protection filing cabinet with index card inserts, that has options for various locks, is rated for 30 foot drops, and is sealed against potential water damage. They offer both four and six drawer options with the larger clocking in at a massive 863 pounds. With each of the 18 sections on the 6 drawer model capable of 25 15/16″ of storage, this beast should hold about 64,800 index cards.
The rough news is that this king of cabinets, while providing great protection and security for your zettelkasten, runs a fairly steep $6,218.00. Despite the initial sticker shock, keep in mind though, that it should provide a lifetime of secure and worry-free storage for just under 10 cents per card.
Unless you’re into the older vintage wooden boxes which aren’t very good for protection against fire or water damage, there aren’t too many modern card index filing cabinet manufacturers out there, and this may be the most solid of the group. I’ll add it shortly to the ever-growing list at The Ultimate Guide to Zettelkasten Index Card Storage.