A good idea for emergency eyeglasses made from cardboard. In photographic terms this works by basically decreasing the aperture of your eyeballs. That has the effect of increasing the depth-of-field, which helps to compensate for your eyes’ faulty focusing hardware. Pinhole cameras, which have huge DOF and no focusing capability at all, use this same principle.
Inuit-like slitted visors used to protect against snow blindness are sort of similar. They reduce the amount of light coming in to prevent snowblindness. A variation on that design was also used by the military to protect against bright flashes from bomb tests. (And by the Mole Man, of course.) I suspect they’d work great as emergency sunglasses too. But if you’re trying for emergency vision correction, I think this pinhole method will work better than a slit.