Balloons and/or Condoms
What if you don’t have a lens? Well you can make your own lens by filling a condom or a balloon with some water. Tie off the end of the balloon/condom then form it into a spherical shape. Inflating it too much will affect the focal point of the sunlight, so don’t overdo it. Squeeze the balloon into a shape which will generate a sharp circle of light. For a condom, squeeze it at the middle to form two lenses that are smaller. Since these have a shorter focal length as compared to ordinary lenses, hold them closer to your tinder.