Generally we would recommend an on-metal asset tag. These are flexible labels with an on-metal layer so they can curve to the surface of the extinguisher but also work on the metal casing.
Some customers also use NFC cable ties attached to the top of the extinguishers or even disc tags attached using a separate cable tie. However these need to be attached carefully of course in a way that would not interfere with the use of the extinguishers.
Reader - 13 Jan 2023