Two ways of improvising sunglasses
1. Make Slit Sunglasses
Slit sunglasses work by limiting the upper and lower angles of light coming into your eyes.
To improvise these glasses you can use the cover of a paperback book, plastic cards or some other stiff but bendable material. Make the slits with a penknife. The nose and earpieces can be constructed of string, rubber bands, twist ties or the material itself.
2. Make Pinhole Sunglasses
Pinhole sunglasses work by limiting the total amount of light that come to your eyes. However, due to the pinhole effect you still have a full field of vision, although you may have some visual artifacts where the two visual fields meet. Pinhole glasses also have the advantage of correcting your vision, also thanks to the pinhole effect.
You can make pinhole glasses out of the same material as slit sunglasses and use a safety pin or bit of stiff wire to make the holes.