[dredmorbius]

Making the device something that is used often is actually a good way around that problem. It's a bit like housekeys. It's not so much that you can't leave without them, but it's a bit hard to return, so you tend not to leave without them....

I'm strongly partial to a worn form-factor. A near-field-chip ring, essentially a modern signet ring, which interacts with various authentication systems, strikes me as attractive.