When i was doing strict keto to lose weight, prior to any diabetic diagnosis, my go to snacks or indeed lunch box was roasted crispy skin, very salty chicken thighs, cold sausages or bacon, with Hard boiled eggs and full fat mayo'. and a pack of pork scratchings. Once i'd found a recipe for very low carb bread, i was then in a position to make either sausage and egg or bacon and egg sandwiches.
If your desperate when you get in cut cheese into sticks and wrap with cold meats, with any low carb dips they can be a life saver.