These credit card sized Fresnel lens are easy and convenient to use. They fit in a wallet and are flexible and durable so you can always have it with you. If you have direct sunlight you have fire.

A Fresnel lens works by focusing all the sunlight passing through it into a tiny, burning spot. Magnification power has nothing to do with the intensity of that spot only its distance from the lens. What does affect the intensity of the spot is the size of the lens. More surface area means more sunlight available to focus.

These have plenty of surface area to produce a blindingly hot spot that will smolder most natural tinders which can then be blown into flame.

This is very old video demonstrating how to light a fire with a fresnel lens... and rabbit poop.