In the spirit of the best camera is the one you have on you right now, this DMT hone fits on a keychain.

https://www.dmtsharp.com/sharpeners/pocket-sharpeners/mini-s...

I used to get by in a pinch with the bottom edge of a ceramic coffee cup but it’s nothing compared to having a properly flat and consistently abrasive stone on you. It struggles with anything longer than a steak knife but most times it’s a steak knife that needs licking up to an edge at the table of a well meaning but poorly equipped restaurant.

I keep meaning to impregnate the inside of my belt with diamond wax so that it too functions as a field strop, though it’s a bit harder to be subtle in taking one’s belt off in public.