Using a Water Balloon:
The more sphericle, the better the lens works. If the balloon is too big, it becomes a squashed sphere and it's focal point becomes distorted, like a line instead of a tight circle. Sometimes just giving the balloon a squeeze in the right axis will bring the light back to a nice tight circle.
Use good tinders. I'm using Fomes fomentarius, false tinder fungus.
Using a Condom:
I'm sure a latex glove would also work. I've used Saran Wrap draped over a wide mouth coffee mug. Just add water and twist up the corners to make a water balloon lens. You can pop a dark air filled balloon with the water balloon lens. I think clear animal intestine could also be used.
Setting the water balloon on top of a coffee mug in the freezer for a few hours will freeze the outer surface of the water, with the center still liquid. That way it will stay clear. Now peel off the latex and use the ice sphere to start a fire.