For a lot of calories in a dense tasty slab, look in supermarkets for Bakewell tart flapjacks, great emergency food.
you can diy your own electrolyte drink powder very cheaply, it's just sugar and salt then add to water/juice/squash etc. You can get little plastic double ended spoons for measuring if you want, they're designed for medical kits to treat diarrhoea, but it's about half a tablespoon of sugar to a quarter teaspoon of salt. If I'm working in hot areas, I make up a tub of it with some flavoured vit c powder and then just add to water as needed.
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