Should be some deals on at least the Stack-On sheet metal cabinets today. Not going to slow down a serious thief much, but they should be good against kids and visitors with inadequately formed ideas about boundaries.
Barring that, a keyed door lock is about $20-30 at Home Depot or Lowes and takes fewer minutes to install.
I "might" know where one of these could be obtained. It's the extra-rare light LEM, but is chambered in .40.