Emergensee Variable Focus Eyeglasses adjust between -6 and +3 diopters, and come in a pretty sturdy case. They don’t correct for astigmatism.
These were apparently distributed to victims of the tsunami in Japan a few years back. I’ve kept a pair in my first aid kit, and on they have been used by a friend who lost his glasses over the side of the boat. He was amazed that we could fit him out with an adequate substitute.
The insert is clearly marked to be used for temporary use only, and are not scratch resistant. On their website, they now have notices posted referencing several states whose optical industries have felt it necessary to inform consumers that these are not a substitute for getting your eyes checked.
I heard about these on a radio program in 2012, and immediately contacted the company, which is in the U.K. They were not selling in the US yet, but made it possible for me to buy a pair.
[Take a look at the Amazon reviews before ordering these. Many people say they are shoddy. However, they sound like they might be useful in an emergency. In the meantime, I'm waiting for Eyejusters to launch. - Mark Frauenfelder]